Don't trust him. You already don't. You used to, and he cheated on you. Twice. Now you have an ex contacting you and telling you straight up that it's still going on. So now you trust him less. This is why giving cheaters a second chance is always a stupid f***ing idea. They cheat again. Every time I gave a cheater a second chance it ended badly. So I stopped giving them second chances. My friends have tried giving cheaters second chances, and they got cheated on again. People on this site have often said that when they give their cheaters a second chance... you guessed it. (Frank Stallone) They cheated again.
Simply put, cheaters DO NOT change. They cheat. They cheat. And... they cheat. If they cheat once, they have broken all trust you have given them, and trust never fully returns. Look at you now. You don't trust him, and every time he leaves I bet it kills you inside. You're left wondering, "Will it happen again?". You've been left paranoid, and possibly a little jealous, and that is NO PLACE to be in a relationship.
Sure he's sorry, sure he feels bad. Still, he can live with it. He's not sorry enough. I find that if you ever give a cheater a second chance, they're relieved. I mean put them through hell, sure, they'll take it. But that's a penance they themselves have prepared for in their own mind. Once it's over, they move on, they do it again, because the fact that you ever forgave them tells a small part of their brain "It's okay.", it tells them "I can still get away with this." and so they do it again. Just like I firmly believe he's doing to you now.
Honestly I would dump him. Now. ASAP. You can keep him as a friend, MAYBE, but not if you think you're just gonna take him back. If you think that, then I'd recommend dumping him, cutting off all ties, and blocking all communication and do it now. Maybe give him a solid break up speech, and maybe a dear john letter. But be sure to end the relationship and get some kind of closure on it. Leave him and don't get back together with him. You know why. I already said. (Previous paragraph) You can't trust him. You have no reason to trust him anymore. His actions indeed do speak louder than his words, no matter how sorrowful they SEEM to be.
Now, this is my opinion. This is my advice. This is how I always react to "He cheated on me. Should I dump him?" because the answer is ALWAYS, undeniably so "Yes! Leave and don't come back. Don't even look back." and the reason I hate answering these questions, is because they never listen. Their immediate response is always "I can't do that..." and why? "Because I love him!" or because he says he's SO sorry. Like nobody would ever lie. Pfft. Even children know that trick. Start crying, make the puppy eyes, talk about how sad, lonely, and sorry you are, and hope that they have mercy
Sure his ex is a undeniably a psycho, but your boyfriend is also untrustworthy. I think they deserve each other, and you? You deserve better.
And believe me, you'll find it.