My wife has bought vests for the little ones she teach's swimming lessons to. She had the ones where the neck collars are supposed to flip the child onto their back to keep the face out of the water, to her they really bite. If the vest you bought him has the neck collar that's supposed to roll him over get rid of it, most don't work.
Take that vest back and keep buying until you find one that works right. And make sure he has swim lessons too, and even after still use a vest.
Fatman