We had a similiar problem. Some older books are good ( e.g., Bobbsey Twins). Also Little House on the Prairie series (people think of them for girls, but my son loved these), Cam Jansen, Encyclopedia Brown, Dragon's of Blueland.
Hope this helps. It is very tricky to find appropriate content. My son would fall apart over anything scary or supernatural at that age (he is fine with it now). I did find that books written 40-50 years ago worked very well. My local librarian helped us by looking for older books on topics my son would find interesting ("Wild West," for example).
