I know you have gotten a lot of feedback, but I had to give my 2¢. My daughter's birthday is August 6... I put her in Kindergarten the year she turned 5. She is very mature, makes straight A's, and gets along well with others. BUT... there are many times I wish I'd held her back. More as they get older, they are exposed to things that I wish she was one of the "older" ones.
Right now, she will be 16 in August. She is the absolute LAST one of all her friends to drive -- she hates that. Plus,she also has friends who are already 17 -- the kids who had summer birthdays that their parents held them back.
When she graduates, she'll only be 17. She has already told me that she may not want to leave to go to college after graduation due mainly to the fact that colleges require them to come early and she'll just barely be 18.
If I could do it all over again, I would have kept her at home (in preschool) for another year.
AND, definitely if he's a boy!