They're called LEGOs. Just get him a ton of basic building parts. They're the universal toy, and furthermore they're great for encouraging him to use his imagination.
I really don't like the fact that so many toys already have the 'activity' or purpose defined in the toy. Lego's let everyone decide what they want to play with, they're good for groups, and they're pretty universal, so you can keep buying Legos and they all go with each other.
Hell, I'm 26 and I still play with Legos every chance I get!