My son was in a similar situation. I put him in Martial Arts, of all things, and he excelled at it. You have to shop around to find a good instructor, since many of them just take your money and hand out belts. But if you can find one who takes the time to teach, it's been wonderful for my son. You can progress at your own pace (if you don't get that belt this time, you can always do it next time), there are constant small goals to reach, and they lead to a bigger goal (black belt!) It also teaches discipline and a good work ethic, which are often difficult to learn for a child who breezes through school with little effort. It's also not direct competition between one kid and another - which helped a lot until he started to get the hang of it. Once he did, I allowed him to start going to tournaments. He now has several shelves full of trophies and medals, and recently told me that he is planning on becoming state champion for his age division.
It's not for everyone, of course, but it has worked very well for him - and there are also several other kids in his class who I suspect are gifted as well. So the added bonus is making friends with "smart" kids of various ages - so he doesn't feel like a misfit.
