I just caught my cheater bf, what to do now?

First of all, don't jump to conclusions. Ask him about it. Why would he be honest to a complete stranger online? He could have very well have been making it all up and just been being stupid. You don't really know that he cheated on you. Maybe it was the truth, but if you like him and you say you trusted him, then give him the benefit of the doubt before you dump him. If you have no other reason to believe he cheated on you, then I'd trust what he's telling you over what he tells a stranger online.