I wouldn't hold him back unless you had some very specific social concerns, because among the gifted cohort, what often appears as social issues ends up being identified as a mismatch where the gifted child is more socially advanced than his age peers. Holding him back and making him the oldest child in the room would make this problem worse.
This is coming from a Dude who may have been registered early for kinder (by one day!) and comes from a long line of late-developing males (I was often the shortest boy in class until a big growth spurt beginning at 15). I often had friends 2-5 years older, and almost none younger.
