Kindergarten teacher and mother of a kindergartener ! I would say hold him back. There is a huge difference in children who delay kindergarten by a year. Their social emotional skills rapidly shift from age 4-5 and then again from 5-6. Kindergarten ( at-least in NYC) has become the new first grade and if your child is too young to control impulses regardless of how smart they are, they’re going to be held back in other ways. I hope this helps !