You could be awesome and say you want to make one of his fantasies come true. The best gift a girl can give a guy is pretty much always just saying 'you can do whatever you want to me' or 'i'll do whatever you want.' Doesn't get much better.
But what he wants might be something you don't want to give to just a benefits dude. If you're thinking of a basic way to make his christmas special, blowjobs are always wonderful, deepthroat is even better if you got the skills.
If you only want to buy him something, don't get some useless girl stuff that is supposed to mean something or make him think of you or whatever. It's not a relationship, neither of you are in love, so take everything romantic off the list. Just stick to something either sexual or friendly.
Stick to stuff you see him do or use or know he likes. Get him a cool movie he doesn't have yet (something funny or with awesome action, like wedding crashers or superbad or saving private ryan or sin city). A cool shirt of his favorite band is always nice (there's some awesome jimi hendrix t-shirts in particular). Could always get a subscription to a good guy's magazine (maxim etc, playboy, any sport he likes, any car stuff he likes).
Regardless, if it's a friend with benefits just keep it real and don't overdo it. You don't want him to worry you're getting too attached (unless that's the idea, but christmas isn't a good way to convert him to boyfriend material). I'd say keep it 50$ or less, make sure it's something very guy-ish, just to say thanks for all the good times dude.