Wow, thanks for all the feedback here!
To be clear, my son is not at pre-school for the academics. We feel that it is important for him to experience a social atmosphere, and although he is ready to learn at a kindergarten or 1st-grade level, I don't know that putting him in a class with children much older than himself would be beneficial.
I've never felt there is "something wrong" with him, but it would be good if he could operate within the norms of a classroom.
I would like to homeschool, but my wife does not feel up to it. (I'm not putting her down at all, it's a big task and requires buy-in from both of us)
For academics, I continue to teach him at home interactively, and he also loves self-directed study.
The real difficulty for him is listening to the teacher, and not running around when he should be sitting, not screaming when he doesn't get his way, etc. At home, if he has something interesting to work on he can sit and concentrate easily for an hour or longer.