Convince him to stop sending half his paycheck to some fat dude who sits behind a computer overseas. Convince him to help up more around the house. Convince him to pay back the debt he has to Chase. Convince him to open some kind of savings account. Getting a CC to save $3 a month on gas isn't going to do anything for him.
But to actually answer your question. Try showing him the awards and signup bonuses you get. Everyone likes money, and if they see they can get more money by using a CC, they might.