Aviation museums, airshows, history books and BAK theory texts are all good starts.
At 12 I'm guessing he likes computer games. I know Flight Simulator kept my interest up during high school, taught me a thing or two and allowed me to meet and talk with some real pilots who I still hit up today for advice. Just make sure he realises that at the end of the day it's just a game. You don't rock up to the Army thinking you know it all from Call of Duty and you don't rock up to a flying school as a Flight Sim expert!
