I'm sorry you have to go through this.
Is hospitalisation for him an option? It will allow him to get onto meds much quicker. While BP makes relationships harder, and requires more effort from both parties, it is not an excuse for anything.
Yes, I had an affair when I was unmedicated, but since I am on meds, have been dx'd and realise what went on, I am a different person (And am back with my boyfriend). I do sometimes lash out at him, but we both know that it's not 'ME' - my bf takes it with a pinch of salt, and I realise I need to regroup myself.