I really like pelhams for stuff like this, with two reins. Mullen mouth with just a little bit of a port so there’s room for the tongue.
Might also be interesting to try something like a Mikmar. While still a snaffle (sort of) it is different enough that it might get his attention without being harsh. Given what you’re saying about his “brainwashing,” something like a Mikmar might change the conversation enough to get him going down a new path mentally.