Susan Cooper's _The Dark is Rising_ Pentalogy
John Christopher's _Sword of the Spirits_ Trilogy
Hal Gordon's _Divers Down! (Adventure Under Hawaiian Seas)_ (available as a free audio book last I checked at feed://www.uvulaaudio.com/kids/Kids.xml )
H. Beam Piper's Terro-Human Future books available in an omnibus edition here:
https://www.mobileread.com/forums/showthread.php?t=47980
Girl Genius is fabulous if he likes graphic novels.
William