You simply have to let him get on with his delusion or you too will start to suffer.
There is no evidence or logic or reasonable explanation that will make him see the truth.
He doesn't want or need a villain in his life and I don't want to be anyone's villain.
He has a villain in his life.
It is so very frustrating and shocking after so long a relationship that it is you. Of course you want, and in fact need, to fix this, to make him see sense and listen to reason. That is not going to happen.

Nothing that you say or do or show him will make a dent in this delusion. This is a horrible position to be in but you have no choice but to refuse to discuss it if he brings it up on grounds that it is a symptom of his MH problem, or tell him you will not discuss his delusional accusation. Walk away if necessary. Don't bring it up yourself.
Let his psychiatrist work on him.
You are entitled to set very clear boundaries with him as to what you will listen to in the meantime. Please do this.
But stop trying to reason with him. You will end up finding your mind focusing on this far too much, crying with frustration, losing sleep.