Does he like mysteries at all? Classics like Agatha Christie are often good forays into "adult" books for this age. Also Tony Hillerman's mysteries, which contain some interesting cultural and regional-specific information which could be followed up with interesting nonfiction books.
Has he read any Robert Louis Stevenson? What about a nonfiction adventure tale like "Endurance" (about the Shackleton voyage)?
To Kill a Mockingbird might be great at this age, but obviously with parental guidance to discuss the more disturbing or mature themes. My father read it to me when I was only 7 and it was OK.
I second Mark Twain, though again with parental guidance on certain language/themes. Definitely second Ender's Game and sequels/relatives. And I'm assuming he's already read The White Mountains and sequels.
