Play some metalcore stuff with him. Believe it or not, most metalcore songs are just super simple power cords in drop D or C. Try some As I Lay Dying or It Dies Today, most of their songs you could learn in a day. I go to
http://www.ultimate-guitar.com/ d/l some tabs and just practice. If it was me, I'd just play the Greenday songs with him, I hate Greenday, but hey it's your uncle, how many people have a cool uncle who plays in a band...not me.