These are not fantasy, but my son loved the Hank Zipzer books at that age. They are written by Henry Winkler. There are nearly 20 of them, so they kept him busy for a while!
Eva Ibbotson also has some fantasy books that are very well-written. Some are written for an older audience, so be careful. A few of our favorites are Journey to the River Sea, Island of Aunts, and the Dragonfly Pool.