Has he read all the Judy Blume books, like Tales of a Fourth Grade Nothing, Fudge, etc? If he likes Ramona, he'll like those. The Mouse and the Motorcycle series might also be good (by Beverly Cleary as well).
My daughter enjoys books by Andrew Clements (Frindle, etc).
I also have a list of books that probably appeal more to girls than boys that I could offer up.