I think dandylion's plan is worth trying.
And if it doesn't work...you might have to re-examine the possibility that you are the one to leave.
i know you do not want to do that. I get it. I really, really get it. But you cannot make another person do what you want. We don't have any magic words. They do not exist. Taking the reins of your own life might mean doing something you don't want to do. In fact, it definitely will, because you will almost certainly find yourself having to choose between living with him or leaving.
Living with him is, as you say, untenable. Leaving is full of unknowns. It's not a great choice. But accepting that that might be your choice will yield more momentum than trying to find a way to make him do what you want.