A kiss, a hug , tell him how much you love him. Guys aren't that hard to figure out . My dad once told me treat your loved ones like they don't know you love them . You have to tell them. People like to feel wanted and loved . I'm sure he will appreciate anything you get him because he loves you. Just get him a card. If he said anything clever or funny when you guys met incorporate that into a gift as well .
Well, my brother's turning 18 tomorrow and I got him a laptop holder cooler fan thingy, some bungee cords, and I'm gonna make a giant huge chocolate chip cookie and my parents are giving him a hundred bucks and his wisdom teeth pulled yesterday.... Sooo.... It really depends on what he's into. You could get him a gift card to his favorite place, a new pair of shoes, a couple hats, some shirts or hoodies, etc. ;o