It's vital for him there's an outside causal influence. It's doubtful, given his beliefs and background that it will be recreational drugs. But what about so-called prescribed drugs? I'm sure the search is already on, but for no other reason than thinking outside the box, I'd have a look at:
and has a sideline selling weight-loss products, including a “shake mix that tastes like a cake mix.”
It seems too sudden to be 'mental illness' per se, and it would almost be a blessing if he goes down with something like viral meningitis . . . anything that's explainable and from which he can make a full recovery. His life as he knows it will be over if nothing is found.