I've heard that Heinlein is great for that age. I'm still trying to get my 11 yr old to read him, but you know how that goes-- If mom thinks it's good, it can't be
He's an advanced reader so it's always hard to find something challenging that he would enjoy, but that's not too mature for him. Although, I'm not sure if he'd like Heinlein. He loves non fiction science, but his fiction tastes seem to be on the silly side (he really enjoys Terry Pratchett)