My 12 year old son likes Origami. I also like the idea of science stuff because my son really loves science, too.

There are always brain teasers, too. Give him a mystery to solve. When I was in a gifted program in school, they had things like that to work on. Since I don't know his intellectual level, I'm not sure what he could handle, but something like that is appropriate for someone who isn't gifted because he could work through ideas at his own pace and could get help from you. I'm too tired today to figure out where you can get these resources, but I'm sure they are pretty easy to find.

Maybe give him a large goal to design an activity for the younger kids where it is all him doing it with you approving it one step at a time.

I also wanted to say how lucky he is to have you. Most people in your situation would just expect an older child like that to entertain themself instead of trying to really work with him.