Well, we're homeschooling. That's certainly not the solution for everyone, but it is what we did. It helped his behavior and mood tremendously, and he is learning a ton.
I must admit, I get very annoyed by schools that try to diagnose and medicate kids willy-nilly. ADHD is a very real problem, and medication is a great thing for some kids, but ruling out GT boredom before passing out the Ritalin seems a pretty obvious thing to do. <Grrr!>
One of the the problems with HG+ kids is that schools just aren't always equipped to handle them. If further grade skipping isn't desired (and it sounds like it might not be for social reasons), then you can push for subject acceleration.
Is there a GT school near you? That might be another option. Even if most of the kids are MG-HG, at least the acceleration might put him closer to a good social fit. Some work for HG+ kids and some don't. (Some GT schools are actually less likely to work with you to meet the child's needs than the public schools are, at least according to our family consultant at DITD. That surprised me!) And they usually cost an arm and a leg. But they're at least worth a look if there's one available.
If all else fails, afterschooling might help to at least be sure that your son is challenged sometime during his day. Many people here do this. It can be hard on some kids because they're already in school for 7 hours or so, and *more* school when they get home doesn't always appeal, even if it's challenging. But for some kids, it's a real sanity saver!
I'm not evangelical about homeschooling, but if I may, I'd recommend that you at least look into homeschooling. As someone who thought I'd NEVER be able to do it, it's significantly easier to do than I thought it would be. If you find a good homeschool group, it can work even for an introvert like me. The social stuff isn't nearly as hard to get for my son as I thought it would be, and the education he's getting is much, MUCH better! More importantly, he's happy again. It's worth a look.
If you have questions, let me know. I'm happy to help, and there's no such thing as a dumb homeschooling question.

There's a lot of misinformation out there. Contrary to popular belief, we're not all religious wackos or hippies.
