Generally I'm in favor of exorcising demons. It's unusual that it's restricting him but not completely taking him over, though. Does he have to [...] resist its effect?
I don't suppose any of your research came with you. I'd want to read over it all, as well as do my own research in the lore I have available in the bunker. I have no idea if any of the exorcism or containment spells that I have access to would work on a demon of another world, but I'd be willing to lend any expertise I have.