Have you tried the "Diary of a Wimpy Kid" series? My son loves them, and I think they are pretty funny too!
I promised my son a new video game after he had learned all his multiplication tables (being able to answer without a pause using flash cards). He learned much faster after that! I don't like using rewards generally, but I figured it was something he really needed to know and it wasn't easy for him. I also got him to consolidate them by doing lots of multiplication and division problems -- he didn't mind that so much.
