Yea For Reading!!!
Encyclopedia Brown - nice age appropriate series, each chapter is it's own mystery so it a bunch of short stories with clues to solve.
Stink - Judy Moody's younger brother
Amelia Bedilia books are really funny (not sure of spelling on that one)
Henry and Mudge series - age appropriate but may seem too easy now that he has read Magic Tree House
Magic School Bus (has tv tie in, but worth it for educational value -- lead to lots of cool home experiments)
Freckle Juice - Judy Blume
Flat Stanley is a nice series
Authors:
E.B. White
Beverly Cleary
Also, Secrets of Droon is a nice big series, but may be for next year.
Go to the local library and ask the librarians, they love to help young readers.
Good for you for raising a reader!