My daughter was there 2 years ago. She enjoyed the Geronimo Stilton series, but we got tired of buying them because she read them in two hours or so. She started with Chronicles of Narnia this year and really enjoys them, except she keeps checking out other books from the library and has 2 or 3 books going at a time...
BOTH my kids (and one is a, well, now he's a 7th grader in the fall, lol).... LOVE the Diary of a Wimpy Kid books, and they have loved them for the past 3 years or so.. as long as they have been out. Also, there is a batch of books that are something like "Campfire Wienies" or something.. that are collections of short stories. They are funny and a little "scary" (really bizarre, more than scary imo), but my kids like them. The one we currently have checked out from the library is "Invasion of the Road Weenies and other warped and creepy tales", by Lubar, David.
Oh, and if he is into humor, my kids absolutely loved every Wayside School book they got a chance to read.
http://www.louissachar.com/Wayside.htm