Harry Potter, if he has not yet read it. For lovers of fantasy I recommend The Chrestomanci Series by Diana Wynne Jones. (
http://www.goodreads.com/series/43763-chrestomanci) Unfortunately this series is not in the AR system. My son liked the Magyk series by Septimus Heap at about that age. A Series of Unfortunate Events by Lemony Snicket, but only if you son likes that kind of humor. The unfortunate Events have a fairly high lexile level compared to their length.
One suggestion is to have him look through a selection of Newbery Medal books and runners up. Most of these books would be at your son's reading level and are excellent literature on top of it.
Whittington by Alan Armstrong
The Graveyard Book by Neil Gaiman
Holes by Louis Sachar
The Tale of Despereaux by Kate DiCamillo
