Do not engage. Walk away. Give him at least half the length of the relationship to get over it. My last boyfriend, 3 years ago, just got out of a relationship when we got together. It wasn't bad per se, but he ended it because he didn't feel he was ready for another relationship.
Like a month or so after we broke up I think he ended up with one of his students. One of his female students. Never date a bisexual. Sorry bisexuals but y'all are heartbreakers

But no really it was messed up