It's like an addiction. He doesn't necessarily love her, but there is something about her that is addicting to him. Think of any addiction you have - chocolate, sex, anything. Chocolate is nearly impossible for me to stay away from.
He needs to have the addiction broken, and it's not happened yet. He's struggling a lot - and you need to make sure that you and the home is inviting. Don't push him away now. Ex-b had said some pretty awful things to me last year - but last week he apologized and said that I'd always been there for him. I don't know what will happen in the future, but "being there" is really important - and if you do it, you'll be less likely to lose him in the long run.
Heck, stay in the cabin that he stayed in last year and take photos - and somehow make sure that OW sees them. Or at least knows you are there, and in which cabin. Put yourself in the position of power. I did that last year - and it helped me so much.
