1. He can try persuading the agent in KUL to check through his baggage to JFK ( assuming KUL-SIN is an SQ ticket as well). This is not guaranteed depending on the station or the agent itself.
2. Otherwise, since SIN is opening to all, as long as he can enter SIN, then its a simple rechecking bags and getting JFK bound bp and SQ check in counters. ( Ensure buffer time though).

Hope this helps.