he's said something twice over the past couple of days that was confusing to me, and I'm not sure how to respond, so I haven't. He said maybe it would be better if I could tell him if he contacted her again it would absolutely be over and I would kick him out of the house.
He wants you to be mommy so he can blame you when he's unhappy or messes up. Stay well out of the way of this. He's in the house because he's choosing to be, right now. You're not kicking him out or making solid plans to leave him, right now. Those things speak louder than anything either of you say at the moment, so you may as well just duck out of these conversations.
You can decide what your lines in the sand are (and deciding there's actually nothing he can do that would make you kick him out is entirely up to you - and it is what you're communicating right now with your actions - and that's a perfectly fine choice to make if that's the truth of where you are) but there's no point discussing them with him. Boundaries are things that you enact, not that you describe and discuss and get negotiation on and agreement for in advance.
I have a boundary around keeping confidences. If a friend breaks a confidence, I never tell them anything personal again. But I've never said to my friends 'by the way, this is how I do friendship, here are my rules, just so you know' and when I have drawn that boundary, I've never said, 'right, now the terms of our friendship have changes and here are the topics we will no longer be discussing. Is that okay with you?' - I just live it. I think boundaries work best (for me) when they are totally silent.