K.P.
Back when I started K I was only 4! My b-day wasn't until Nov. I think it depends on the child. It never made any difference to me.
If he is academically ready you will do him more harm if you hold him back. Remember in K through about 3 grade the teach to the lowest level. Meaning they teach to the child who knows the least. So if you wait, and next year he ends up being way ahead--then what??
My sons bday is in Jan so he could not go to K when he was 4. I wish he could have. He was ready to read, doing simple math and loved science. Last year when he was 5 I home schooled him for K because He was too far ahead to go to K, but not enough ahead for 1st. So this year he is in 1st grade. At the beginning of the year I would ask him "what did you learn today?" And he would say "all stuff I already knew" SO now I just ask "so what did you do?"
Send him to school. Go by academically ready not age. I wish the school had let me with my son.