It seems like you both should consult a therapist, best of all if together. I think you should heal together. It's obvious that he isn't equip to deal with this problem, and he has to learn how. I believe very few of us would actually react correctly to something like this. Looking for a sort of justice is actually a very usual reaction, I would think. It is silly that he wouldn't trust you because you dated someone else while you were separated though. However, therapy can help with that as well. He can be much more supportive once he knows the correct way to be.
However, if you've already broken up with him, I think the problem's solved on behalf of the relationship if you already have. You could convince him to see a therapist by himself, but it's up to you if you are already separated, in which case, you aren't responsible for helping him with that.
If you haven't, I don't think people here on Gaia can tell you more about your situation than you can and especially that you should dump your boyfriend based on the little information that you can give without revealing too many personal things, and without projecting themselves onto you. It seems that people are all too ready to throw something away for a few cracks rather than attempt to fix it, especially something that isn't theirs. We can and should only listen. Nobody here's an expert, but we can advise you to consult an expert. However, even if you decide not to stay in a relationship with your boyfriend now, this can still be a growing experience.
TL;DR: my two cents is that if you're still together, you should see a therapist together. It will help with the healing process.