First, I think you should put an extra-large stuffed bear in your dashboard, and maybe a romantic-looking card.
And...you should focus first on yourself. At the end of all this, you don't know if you'll have your H. You do know that you'll have yourself and your son - so make sure that is your first priority. I have read that MLC's can take up to 5 years to end, and he is definitely not thinking rationally. So I'd be firm when he says something like, you weren't interested - and let him know that you were.
If it was me, I'd have kicked him out of the house by now. It'd be a risk, but the whole thing is a risk. He needs to get her out of his system, and if he was with her all the time, I think that would happen quicker. Of course, he could end up with her - but that could happen anyway, even if you hang out and let yourself be tortured over this for several years. Men often want something they can't have - and you are bending over backwards for him, and *she* may be the one he is longing for. If you kicked him out, eventually that might switch.
