Interesting that he is reading those, as I would consider many of those on the "scary" end of the spectrum for kids books (esp. Roald Dahl and HP). If he is okay with those, he is probably okay with most anything that would be recommended out here. There are some other book threads that you might find useful:
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/102996/1.html (started as a thread for 10 year olds, but there are some possible recommendations in here)
http://giftedissues.davidsongifted....e_books_you_read_when_you.html#Post98082 (this is a nice thread, has some suggestions of books that have been around for a long time but are still great)
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/91730/1.html (there might be a few suggestions in this one)
