Hi, J.---
This is a tough question. I think what is most important to keep in mind is how emotionally ready you think your little guy is. You said he is friendly---being overly shy would be a huge concern for me. You also say you feel he is academically ready---also very important.
Kids are so different.....my daughter has always been very mature for her age, but my son is not (he might be right on par for his age---but, compared to her, he's always seem a little immature). Chances are quite good some of the boys in his class are going to be less mature than him---and certainly less prepared academically---so, in my opinion, his age is the least important factor, and I would send him based on that.
But that still doesn't make the decision any easier for you. I would recommend making an appointment to sit down and talk with the person who would be his teacher, if you can. Express your concerns, and get input (s/he should be somewhat of an 'expert' on kids that age, I would think!) Take your son with you---allow him to see the classroom---see how he reacts to everything in it. Then make your best decision with the information you have----and remember that if he seems to be struggling after school starts, you can ALWAYS take him out and wait till next year.
But, J.---the fact that he has a Mom observant enough and concerned enough to ask these questions makes me think he's going to be JUST FINE!! :-)