I'm of South Indian decent but my Grandpa moved to Singapore during the 1930's to start a new life there. My dad used to tell me that my Grandpa fought for the Japanese Army during WW2, but unfortunately he passed away before I was old enough to pick his brain in-depth about the subject. All of my dad's siblings have passed since then and I really don't have anyone to confirm the stories that my dad used to tell me about my Grandpa.
I've read that many British, Indian and Singaporean men/soldiers were put into POW camps -- but I haven't come up on anything that talks about forced enlistment into the Japanese Army.
At that time, there was no such legal status as a Singapore citizen.
There were however, British citizens, Commonwealth citizens (virtually all of whom were Caucasians from Canada, Australia & New Zealand) & colonial subjects (mostly Chinese, Malay, & South Asians).
The above 3 groups did experience being forced into slave labour for the imperial japanese army (IJA).
The IJA did conscript ethnic koreans & Taiwanese as cannon fodder. But that was about it.
Your grandfather may have been in the Azad Hind Faij or Indian National Army (INA), set up by Subhash Chandra Bose. The INA was fully sponsored & supported by the IJA and it carried out extensive fighting within Burma itself. Many of the fighters within the INA were former South Asian ex-soldiers of the British Indian Army whom were placed inside Japanese concentration camps as POWs. Many POWs of many ethnic groups and various countries were sent off as slave labour in various Japanese construction projects all over Indo-china during WW2.
Some POWs of the British Indian Army did join the INA to avoid slavery. Others were volunteers.But Subhash Chandra Bose was also highly respected, with a lot of credibility at that time in the South Asia anti-colonial movement. His support gave the IJA a very strong reputation of legitimacy.