S.C.
He is the adult, he received the gift, he should write the thank- yous. If he doesn't, then he is the one who appears to have no manners, not you. However, I don't think many people will think much of it if they don't receive a thank you from him. For some reason, guys get off the hook with this kind of thing. Besides, if you do write and sign the cards, that looks pretty silly, too. It's like you are his mama. If he told everyone "thank you" at the party, just leave it at that. It's not your problem.