Wait, for all the reasons listed below.
Do you really want a 15/16 year old hanging out w/kids who've been driving a while, probably have a beard, can legally vote, etc when he's a senior in high school? Do you want to send him to college (or have him head off to the military) when he's barely 18? Or have him dating girls that are that much more mature?
Ok, that's a long ways off. Both of my kids are in this predicament. My son has an August b-day, my daughter a Sept. b-day. My son is now in 1st grade. We waited and it was absolutely the right decision, no question. He's little for his age, he wasn't ready socially, etc. it is really hard when you have a bright kid - he's reading at a 3rd/4th grade level right now, so he wouldn't have struggled academically, but oh, how he would have struggled in everything else, particularly because he's very small for his age. He's the 2nd shortest in the class, ahead of one girl, even though he's the oldest.
we are also waiting w/my daughter. she turned 5 this september and is in pre-k this year. Again, academically, she'd be fine (she's reading at a end of 1st grade level right now), but she wasn't ready. Much harder to make the call for her as her social skills are stronger and she's tall for her age. But, when we started preschool this year and she hid behind me despite the fact that she knew 3 of her classmates and both teachers, I knew we'd made the right call.
think through all the school years - beyond is he going to be bored in kindergarten? You can always supplement at home. and do the homework to find a pre-k that will challenge him. We are using a preschool called Kids Discovery Days (at 68th and Carr in the Methodist church) that has an incredible pre-k program for 4-5 year olds.
Also discuss this w/his preschool teacher - they typically have wonderful insight since they see the school behavior that you can't. For both of mine, their preschool teacher when they were 4 told us to wait.
don't rush your kid into growing up, let him just be a kid for as long as possible.
Good luck.