Wow, that's pretty vauge... and you are going to get lots of suggestion. But, I've found that
http://www.whatshouldireadnext.com provides some pretty solid suggestion. You put in a book title or author that he likes and it will make suggestions. You can look up those books in Amazon for summaries and reviews and probably find something he wants.
There are also tons of public domain books here that can be downloaded. Has he read some of the Classics? Like Treasure Island, Pamela, Anna Kerinnana, etc.
BOb