Your requirements are a pretty good illustration of why sci-fi and fantasy are so commonly associated with gifted boys of that age. I'd turn him loose in that section and see what happens. It's exciting and plot-driven, and explores a lot of very meaningful themes.

I'd start with these:

The Hobbit/LOTR
Ender's Game
Hitchhikers' Guide to the Galaxy - if he has an absurd sense of humor
a collection of Greek/Roman mythology