Thank you everyone, for your words and support. As far as I know he has honoured his promise and not been with anyone else since we got together, and I've never found evidence that he has. Internet stuff, yes, but not anything else, as far as I know.
It is hard to trust someone when they have lied to you at least once, and I'm not sure what to think about that.
Well, I did my little speech today and he immediately agreed with my terms that he can only have phone/internet chat with this woman and nothing else, because "that was all" he wanted in the first place, was to talk to an old friend for advice.
I'm not completely naive, but at this stage I will take him at his word (and honestly, part of that is the financial issues, and I know how bad that sounds, believe me), because he simply doesn't have the opportunity to cheat. He works in another state, and I drop him to and pick him up from the airport. He doesn't have access to a vehicle, either.
I know if he was determined, he could make it happen, and if it does, that will be it, even if I have to live in a cardboard box under a bridge.
Fingers crossed. I'll be putting some money aside for a contingency fund in the meantime.