What else can you say about him? How is he gifted - math? Language(s)?
Hobbies are great things but typically not something you can pick for someone...
That said, one thing that leaps to mind is a digital camera because almost everyone loves taking and looking at pictures (if he doesn't already have one) and the price range has gotten pretty low for decent quick shots. Photography is a fun hobby even if you don't have time/inclination to go very technical or very artsy-fartsy. It has become even more accessible than ever in my opinion and is a fantastic creative and intellectual outlet even for those pressed for time and/or cash.
