Great suggestions here - we have also struggled with the same issue for years since DD10 is advanced - trying to balance appropriate content with appropriate reading level. DD is so excited to be going to middle school because she had read all the books in our school library by third grade. I agree with the PP who said there is nothing wrong with reading some books at grade level, not always reading at reading level. DD breezes through the 39 Clues series but still enjoys reading them. DD7 still enjoys Junie B. Jones even though they are below her reading level. Sometimes it's nice just to read for fun instead of reading for the challenge (not that you can't do both).
A few I didn't see mentioned:
The Inkheart Series by Cornelia Funke (Inkheart, Inkspell, Inkdeath)
Other Cornelia Funke like Dragon Rider and the Thief Lord
Someone mentioned the Among the Hidden series - Margaret Peterson Haddix has so many books DD has loved - Found, Sent, Running Out of Time, as well as the whole Among the Hidden Series
The Kingdom Keeper books by Ridley Pearson (set in Disney!)
DD started reading Peter and the Starcatchers in second grade (someone else mentiond that) - there are now four books in the series.
Has he read the classics - Stuart Little, Charlie and the Chocolate Factory, James and the Giant Peach, Charlotte's Web?