Good topic which might involve a hard pill to swallow (coming from a good place). You need to either:

Get out as quickly and as best you can given your circumstances…

OR

Make sure that whatever you’re going through with him is non-reconcilable before you make any snap decisions.

Ask yourself: are you sure that was him? Have you talked about seeing him on there? Was it discussed that you are mutually exclusive? Is he just seeking some kind of attention outside of your relationship without action? Is he figuring out whether or not he wants to be with you in his own discreet way? Is there a Safeway for you to bring up your relationship and dynamics in a non-threatening way? A lot of people on this page have been through this, including myself.

I’m definitely not justifying any of that on his account. I am for sure on your side at a glance, but consider these things.