Any of the
Heinlein juveniles would be good (I would recommend avoiding his other works until your son is older). Might even like the
Foundation series by Asimov.
The Hobbit (which I loved) or Lord of the Rings (which I haven't been able to get through...).
He may also like the
Riyria series by Michael Sullivan; it's a light easy fantasy read with plenty of action and adventure.