The Harry Potter books aren't available as ebooks legally, unfortunately.
I adored the Oz books when I was that age (and I still do, twenty years later!). They are all available for free here in the MobileRead library, and there are enough of them to keep him busy for a while.
You might want to check the Project Gutenberg Children's section for more ideas:
http://www.gutenberg.org/wiki/Catego...%27s_Bookshelf