Most Helpful Guy
Get him some kind of bundle, so that you know he at least likes SOMETHING in it. That's what i always do (at least until i get to know the girl better).
I'd suggest getting him a hoodie or t-shirt (if he has a favorite band or sports team get one with that on it), a bottle of cologne that you think smells good (and tell him you think its sexy on him), as well as maybe a gift card, or some kind of food, snack, or candy.
Most Helpful Girl
Do a prank birthday gift give him a baby name book and be like I'm ready ;) and once he naked and stuff give him a sweater and said it was a prank surprise!