What do you do when you like a guy who likes you but is dating someone else?

If he likes you a little, and you like him too, then maybe theres something that could happen. But respect his current relationship - don't try to force your way in, instead secretly stand in line and wait your turn.

Chances are his current dating partner will not last that long. They don't.

Stay friendly with him, keep it social. And one day it may be your turn.

And don't stop looking - you might find a better option than him if you just keep your eyes open.