In May 2004 Manmohan Singh became  India’s  Prime Minister. When a person becomes the Prime Minister of a country, it is  natural for the papers to print pictures of his immediate family. 
When I saw the pictures of Surjit  Singh Kohli and Kuku Kohli, brother and father of the PM, respectively, I  wondered that the PM’s surname too was Kohli (caste Khatri or Kshatriyas) but  for some reason he had decided to omit it. People called him India’s first  Sikh Prime Minister and this confused me even more because neither did my  next-door neighbor Vikas Kohli sport a turban nor was he called a Sikh.
  When I asked my 74-year-old  Punjabi mother about this contradiction she   told me that there was a tradition in Punjab where the first son was  made a Sikh and dedicated to the Army. My next question to her was why did the  Punjabis dedicate their first born son to the Army? She did not have an answer so  did my own research and here is what I  discovered -
The article was written in July  2004 and edited in April 2017.
After referring to four books I came  to the following conclusion. Am willing to stand corrected and would be keen to  know a different point of view.
According to volume 7 of the History & Culture of  Indian People published by the Bhartiya Vidya Bhavan - "Disciples of Nanak  called themselves Sikhs; derived from the Sanskrit word sishya, meaning a  learner or a person who takes spiritual lessons from a teacher. The public  called them Nanak  Panthis or Sikhs. Panth literally means path or way and it has  been traditionally used to designate the followers of a particular teacher or  of a distinctive range of doctrines”. 
"Singh means devotee". However, today Singh has come to mean  Lion and is associated with fighting classes throughout North India esp. in  modern day Punjab, Haryana and Himachal Pradesh. 
Having experienced the strength  of Sikh opposition during the Anglo-Sikh wars and grateful for the assistance  received from Sikh princes during the Mutiny of 1857, the British realized that  Sikhs would be an effective buffer between Afghanistan and India. 
Therefore, the British reduced  the number of Bengali soldiers (many of whom were involved in the 1857 Mutiny)  and replaced them with loyal Sikhs and Punjabi Muslims. It however, insisted  that only Kesadhari  Sikhs could join the  army i.e. those who sported the five k's. The enlistment of Sikhs increased  steeply. Since it was mainly Jat Sikhs who sported the five k's then, they were  the biggest beneficiaries since soldiers were well paid, given agricultural  land and pension. 
Veena Talwar, author of Dowry Murder, the Imperial Origins of a  Culture Crime wrote, “To prevent the kind of mutiny they had experienced  from sepoys in 1857, the British organized religiously segregated regimental  units from the alleged 'martial races', Sikhs, Pathans, Rajputs and Gurkhas.  This severely restricted Hindus of other castes who wanted to join the army,  particularly Khatris, who had served in Maharaja Ranjit Singh's forces. It is  important to mention that Hindus, particularly Khatris, who were acknowledged  as Kshatriyas but were arbitrarily limped together with the 'trading castes' in  the British census reports (since large number of them were educated and  engaged in trade), were seldom accepted into the British military service. The  Khatris, who had also been landholders, acquired further acreage till the  Punjab Land Alienation Act of 1900 forbade them to do so as a  'non-agricultural' tribe. Many families got around this artificially imposed caste barrier by  raising one or more son as a Sikh, chiefly by having them adopt the name Singh  and grow hair/beard to match." 
The children of such Sikhs became  Sikhs and so. Till a couple of generations ago, the same family had two brothers,  one Hindu and another Sikh. 
W. H. Mcleod wrote in his book Who is a Sikh, "Appreciative of the strength of opposition  encountered during the Anglo-Sikh wars and as a result of the assistance which  they received from the Sikh princes during the Mutiny, Sikhs were easily  accommodated within the British theory of the martial races of India and Sikh  enlistment increased steeply. For the British, martial Sikhs meant Khalsa Sikhs,  and all those who were inducted into the Indian Army as Sikhs were required to  maintain the external insignia of the Khalsa".  
“The British enforced rigid occupational  boundaries by creating ‘traditional agriculturists’, ‘martial races’ and  ‘trading castes’. They could not trust the educated Khatri to be as obedient a  soldier as the Jat, and certainly missed the rationale for the many male  children being produced in these families.”12 Note that Ranjit  Singh's commander-in-chief who led the victory of Attock in 1813, Dewan Mohkam  Chandand all Sikh Gurus were Khatris. 
However, not all male Hindu  children who were made Sikhs joined the army. When a Hindu couple did not have  a son or their progeny did not survive, they made a Sukhna - a sort of  benedictory prayer that the first child would be made a Sikh.  This custom was alive and being followed even  into the twentieth century. Sometimes, the Sants of Sikh sub sects like Udasis  and Nirmalas would ask that a son be dedicated to the Sikh cause, if the number  of issues in a family were many. 
This explains why the British  wooed Jat Sikhs, gave them predominance in the army, the rivalry between them  and the Khatris and why Hindus (mainly Khatris) made the first son a Sikh. 
Further in order to safeguard  their geo-political interests the British took various decisions that divided  Punjab into Hindu and Sikh. Please read How  the British divided Punjab into Hindu and Sikh - link at end of this  article. 
The maximum  number of followers of Khalsa were Jats who as we know  are tall, sturdy and big built people. (Jats  are found in modern day Punjab, Haryana and  Western Uttar Pradesh). Because of reasons listed in the preceding paras other  castes like Khatris made  their first son  a Sikh. Sometime around 1985 I remember my Delhi cousins telling me of the  problem this created for the families of two elderly cousin brothers one of  whom was a Hindu and another a Sikh. This happened because one of their  forefathers wanted to avail of the economic benefits offered by the British to  the followers of Khalsa and had decided to become a Sikh.
Impact on Names
Today anybody with a turban has Singh as his middle name  or last name. Two people with the same surnames could be Hindu  & Sikh. Let me explain. 
My first boss Sukhwinder Chadha had a turban. We worked  together in Rajpura, Punjab. Due to the resurgence of Khalsa in the 1980's his  name in inter office memos would appear as Sukhwinder Singh Chadha to show  he  was a Sikh. However, he signed  cheques as Sukhwinder Chadha as that was the  name  on his birth certificate. 
Conversely there is another Chadha, a school friend  Vineet Chadha who is  considered to be a  Hindu as he sports no turban. Non-Jat Punjabis with turban, like Khatris etc  invariably put Singh as their middle or surname to prove they are Sikhs. Here  is another example. 
A girl I was looking to marry had the surname Batra. Once  she went to her father’s place and  used  her Dad’s email id whose name was J Singh (he had  retired from the Indian Air Force). I was  confused and asked whether she was a Batra or a Singh (not that I had a problem  with either). She could not explain why father and daughter had different  surnames. Since her father sported a turban he was considered a Sikh but what  about her? 
Impact in  Indian Armed Forces by Sandy 
“I cannot help but give an input on the Sikhs. This is  based on my interaction with the armed forces for the past 21 years. I joined  NDA at the   age of 18. Before I joined,  I used to think that Sikhs were one community without any caste divisions but I  was wrong. 
After joining the services I realized that Sikhs were more  divided on caste lines than Hindus. The Jat Sikhs consider themselves to be superior than others. The Sikhs who have surnames common with Hindus e.g. Chawla, Arora, Kohli etc. are called Bhapaa Sikhs or the trading (read Baniya) Sikhs and are considered inferior as compared to Jat Sikhs who are basically land owners and farmers.  
Another caste of Sikhs is considered even lower than Bhapaa Sikhs. They are scheduled caste  Sikhs called Majhabi. Their  parental/ancestral occupation was of sweepers/garbage lifters. The British  encouraged this by having the Infantry units segregated on caste lines. When  one says he is from the Sikh regiment, it means he is a Jat Sikh. 
  
  However, when a Sikh soldier belongs to Sikh LI (Sikh light infantry), he is a "Majhabi" Sikh. A Jat Sikh would rather be dead  than join the Sikh LI unit. You will  hardly find any Jat Sikh  officers in Sikh LI regiment. Only Hindu officers would be heading Sikh LI units. That is why the non Jat Sikhs rarely reveal their surnames for the fear of being ostracized / ridiculed in the Sikh community. They always suffix their first names with 'Singh'.
  People of the Khatri or Arora caste need to prove they  are  Sikhs, a Jat is under no such  compulsion. A Jat Sikh has Singh as part of his name e.g. super cop K. P. S.  Gill. Note that the S stands for Singh but is silent. 
To retain effective control over  Punjab, the British accentuated the wedge between land-owning Jats and  non-agriculturists. The Punjab Land Alienation Act of 1900 “enabled the  government to retain its inflexible revenue policies and continue to blame  peasant proprietors’ misfortunes on Hindu moneylenders. It was done to pacify  the land owning classes and deflect a rebellion, and to aggravate and exploit  any tension that existed between Hindus and Muslims to keep their political  grip on Punjab." 
"This piece of legislation  created a favoured, ‘dominant’ agriculturalist class at the expense of other  social groups. Here the ‘agriculturists’ were Muslim tribes and Sikh and Hindu  Jat zamindars, and the ‘non-agriculturists’ were Hindu Brahmins, Khatris and  Banias. The Act made tribe and caste the basis of land ownership.” The British  played up differences between Hindus and Muslims and sought to make Muslim and  Sikh Jats loyal subjects, thus safeguarding their own position in Punjab. 
Peasant discontent  was converted into fresh and deep religious antagonisms that smoldered  dangerously in 1907 and eventually resulted into the flames that ravaged Punjab in 1947. 
  
  The above explains why the first son was made a Sikh and  how divisions in society were created to secure British rule. 
I believe the British deliberately did this to drive a  wedge between Hindus and Sikhs using modern day connotations. The famous  ‘Divide & Rule’ policy.
Also read:
  1. How the British divided Punjab into Hindu and Sikh  
  2. How the British sowed the seeds for the Khalistan  Movement before Indians took over  
  3. History of Sikhs  
  4. How the  British created the Dowry System in Punjab