Keep mocking him. Ask him who's behind it, and come up with more ridiculous secret masterminds. Poke holes in all his "logic", such as asking how it is that you're the brainwashed one when you're the one critically thinking and refuting the outstanding claims being made by him. Really try and get under his skin.
Find out if he's Q-Anon and then mock Q if he is.
After one exposure to me, the resident Q-Anon kooks at my job have learned one thing - never discuss Q and the conspiracies around little old me ever again.