His interest level is probably outgrowing these books. If he devoured the tree house on Amazon rainforest in 5 minutes, he is probably likes fiction but may be ready to jump to more challenging books. Maybe try Geronimo Stilton novels, Encyclopedia Brown (short stories but a little more meaty), or maybe some of the illustrated classics like Swiss Family Robinson...

Definitely enjoy your local library to save your wallet for books he checks out three times or more...