Maybe, the ideal answer would be finding something about one of his favorite hobbies.

For example, if he has a favorite band or singer you could get him a CD that he doesn't already have, or go for the standard mixed tape thing if you wanted to.

You could also get him movies or video games etc. like everyone else said.

If you want something cheap yet heartfelt, maybe a nice pic of the two of you together (in a frame of course), with a special message written by you on the back.

Or gift certificates for his favorite store, or bake him his favorite snack. This will be great for him.