first, I am so sorry you are dealing with this. It's horrible, and I don't want to discount it. That said, a posting on a website does not necessarily mean he's cheated yet. He could be "testing the waters", and just seeing what it's like out there without necessarily acting on it. It is, of course, a giant red flag that there is a problem.
I think likely the best thing that you could do is get yourselves to counseling. You're going to have to talk to him, and it would be best if you could do it when you are feeling calm. Anyway, I'm sorry you are going through this. I can only imagine the feeling in the pit of your stomach right now.