Age is definitely not an issue with any of our kids. DS11 has a summer birthday but went to K when he was a new 5, and he's had no issues because of it. DD8 is now 1.5 to 2+ years younger than her classmates and she's actually much more comfortable and outgoing now than she ever was with her agemates (partly because she doesn't have to hide her abilities, partly because she just is interested in the things the older kids are). In fact, she has a large group of really great friends, both her age and older, and nobody seems to care at all about her age even though they all know she's younger (and, in fact, she's invited to virtually every get-together, even when the guest lists are small). DS6 is the youngest in his class but every last one of his friends is more than a year older than he is. He's very social and no one cares that he's younger. BTW: my kids are all tall for their age, but I just think size is not important. My DD has friends in her class that are 1.5 years older and many inches shorter, but no one ever suggested they hold those kids back because of their size!
As far as starting 1st without the learning-to-go-to-school that K provides, for DS6 it was no big deal. We talked with him before he started about the basics like raising your hand and asking to go to the bathroom and going to the cafeteria for lunch, but these things literally took days for him to learn. Some things we never thought to mention to him and have figured out as we went along. In our case DS was familiar with the school layout because he has older siblings, but any child new to the school in whatever grade would have to find their way around.
In addition to your son maybe trying out a couple hours of K now, you might also consider sitting in on a K class for a bit. I think you'll find a HUGE range of maturity and behavior in a K classroom and, because the kids are the "proper" age, the teacher somehow manages to deal with issues of differing maturity. Unless your DS is very immature, I tend to think that he'd probably be indistiguishable maturity-wise given how close he is to the cutoff. HTH!
