I'd put him in his age group and see how it goes. Presumably with this age group, they aren't going to be doing academics, so fit there isn't an issue, and by no means all very bright children have difficulty playing with age peers. In fact, if yours hasn't yet had much experience in a group situation with other children, I'd be wary of blaming any trouble he does have on his brightness - it may well take him a while to settle down and see how it works, and that's fine; might even be easier for him to start in an environment where mostly parallel rather than cooperative play is what's going on.
