Does he like science fiction, as well as fantasy? If so, he might enjoy the David Brin Uplift series (first book in the first one is Startide Rising. Or some John Scalzi? I guess he sometimes has some sex in his books, so you'd want to screen a bit. My kid loved The Martian at that age, although I did have to have a talk with him about just because we were letting him read a book with the f-word in it didn't mean he was allowed to say it at school. Ender's Game often resonates for gifted kids.
How about Watership Down? Or The Wednesday Wars? (That might get him to read some Shakespeare, too!)
