Schedule in a time to meet the school librarian (with him along) and she can teach him how to browse the sections of the library that has material suitable to his reading level...and how to select material himself. He should be reading a mixture of fiction & non-fiction, material that is too easy, material that is just right and a few things that are a bit harder too. Talk to him sometimes about what he's reading, and read aloud together, so that there's a culture of shared enjoyment in the house.

There's a fabulous website in every state for the Premier's Reading Challenge, they have age-grouped book lists which could give you some ideas. Don't forget that you can also provide him with magazines, newspapers and "real life" reading opportunities. It all counts esp if he's thirsty for information.