It seems as if this might be connected with his odd dreams. Whatever this creature is, it does seem to come to him in dreams. I think that's when it's getting worse. Every time he sleeps, he opens himself up to influence. I mean. It's what I did when I was
We need to find a way to break the connection. A few night sleep on the astral plane might help. Or if you know a wizard who can whip up a bit of dreamless sleep potion. It might... stall things.