My son is 9. He did fine in K at 5 years old, but now in 4th, I wish I would of given him another year to mature. He is playful and very smart, but not ready for the huge burden of work. So, I took him out of school. He was only going part time, now I have him home full time.
My daughter is 7 tomorrow. She is in first grade and doing extremely well. She could of gone into K at 4 and turned 5 in November, but I kept her in Pre-K for two years. She does extremely well. When she went to the second year a of Pre-K, it was Winter Break and she said to me, "Mom, all my friends from pre-k last year are in K." I said, "That's right, but you are very special, you get to stay in pre-k and have Mrs. O and play with all the neat toys for another year." It took her a half a year to even notice and then she mentioned it and dropped it. When she went into K, she was ready.
I say do it. It is a blessing in disguise. If you think the other kids will be a bother, I would try a different school, so he is totally removed from any recognition from other kids.
It is OK, to play and not be ready.
God Bless!
J.