There's a lot of great books out for that age. If they liked Harry Potter, then Tales of Beedle the Bard by JK Rowling is out. Another fun series is the Wimpy Kid series by Jeff Kinney. The Phantom Tollbooth is a classic story in that age. And of course Where the Red Fern Grows.
He might enjoy "The Dangerous Book for Boys" by Conn Iggulden, "Humanity's Edge" by Tamara Wilhite, "The Boys' Book Of Survival (How To Survive Anything, Anywhere)" and the Encyclopedia Brown mystery books.