I don't think it has to be as simple as saying 'ditch the boyfriend'.
It might well be the case that he ends up wanting to leave his fiancee for you, simply because you are offering something she isn't.
You are offering non-judgement, openness and freedom...she is offering him a cage.
It doesn't matter how much he loves her, this will eat away at him over time if he stays with her.
I would recommend a discussion with him about it on which way he wants to go, not in the 'micro' terms of your relationship, but in the 'macro' terms of whether he wants to stay poly or go back to mono.
My hunch is that he is happy being poly and really wants his fiancee to work out her issues, but eventually he will leave her when it becomes clear that she isn't going to work them out.