Good suggestions above.
If he's into fantasy/science fiction, I would add:
The Lord of the Rings trilogy (JRR Tolkien) - if you/he are only familiar with the movies, the books are even better. Also, The Hobbit (prequel). If he's read LOTR, then The Silmarillion might interest him.
The Eragon books might interest him (I think there are four of them now?). The first was written when the author was fifteen and they've been well received.
Anything by Anne McCaffrey - the Pern/dragon books, or the Ship books, or etc.
If he hasn't read the Madeleine L'Engle books (A Wrinkle in Time etc.), or Le Guin's books, he might find those interesting.
The Killer Angels (if he's interested in history - it's about Gettysburg)
Across Five Aprils (another Civil War book)
To Kill A Mockingbird
Little Britches - might be a little dry, but it's kind of the 'boy pioneer' version of the Little House series.
I just read Jefferson: An American Family about Thomas Jefferson's descendants via Martha Jefferson (his wife) and Sally Heming (his slave). It was fascinating, short bits with many different people, lots of photographs etc., and a very interesting examination of the issues of race, family, etc. A good springboard for discussing race, slavery, Jefferson/the founding fathers (the contradictions between what he wrote/believed, and what he did), etc.
I think I read Adam of the Road at that age, and liked it.
Jack London books, if he hasn't read any of those
If he's into sports, anything by John Fienstein would probably work.
He's at an age where he can probably read at an 'adult' level, although he might not be ready for some of the emotional/relational things which show up in 'adult' books. But, if he's interested in them, things like Barack Obama's two books -- or other biographical sorts of books - might be interesting for him. Ted Kennedy's biography was interesting to me, and has so much history, that regardless of one's political stance, it would probably be interesting. Yeager: An Autobiography was one of my husband's favorite books from that age (written for adults but he loved it).
I'd encourage reading some nonfiction with him -- it's a really important skill to develop at this age. So if there's something that interests him (gardening, camping, space) that could be really fun to get him started on.
Hope you find some really interesting books!
Not all who wander are lost.