There is only two possibilities in my opinion:

1. The easier one, but only temporarily succesfull, is to swamp him with B6 and zinc. Until his body and mind absorb enough of it in spite of his condition. That way he will temporarily be sane, and you will get your break, until the next **** hits the fan.

2. You put him on a diet of boiled parsnip and pure water (so, not the water contaminated with residual medicine that comes out of your tap) for at least a week, preferably two, and see him relax. No matter how much he protests/hates your measures. Then, you add any kind of food/herb/condiment one by one, and if you see him having an adverse reaction, it goes out and stays out. This will give you a lasting effect.