My bio son's birthday is in early September and like you mentioned about your son he was ready academically but not really socially. (The cut-off here in Michigan isn't until Dec. 1st.) We held him back and he went through a year of 'Young 5's' and started kindergarten a few days before his 6th birthday. I have never regreted for a second holding him back. He is now about to enter 4th grade and is an excellent student who gets great grades, is loved by his teachers, and very confident. That extra year really gave him a chance to mature and be much more prepared to go to kindergarten and for the years to come.
My younger son (home from Guatemala in Feb. 2005 at age 20 months) turned 5 in late May and we are holding him back as well. He is a wonderful, charming, social child, and everyone at school adores him, but kindergarten is so academic now (reading and doing basic match, etc.) and there is just no way he has the maturity and self control to focus as he would need to. He will probably end up being the oldest in his grade, but someone has to be.
And hopefully being one of the older, and (hopefully) more mature kids in junior high and high school will be good as well.
I have many friends who have held their children back from starting kindergarten for one reason or another, and I have never heard of anyone who regreted it. And do know several people who regret not doing so.
Good luck!