You can't "validate" someone's thinking when he's not in his right mind.
You seem very invested in getting him to see the truth. It won't happen. He's not capable of it, and right now, he's not interested in becoming capable of it.
It's very possible to leave someone while you still care for him. I've done it twice. In fact, I am often grateful I didn't stay until I hated the person.
I'd be working on a plan to move on, if I were you.