Does anyone know anything about psychology? Hubby o's having an issue I don't know how to help😦😦😦 He has this…

Does anyone know anything about psychology? Hubby o's having an issue I don't know how to help😦😦😦

He has this reoccurring nightmare that I'm cheating on him. They used to be just once in awhile now it's every night, even if he gets up and walks around it comes back. He gets really upset because he says they are so realistic. But it can really affect his mood and I don't like it.

I've never cheated on him. Never so much as thought about it. He says he completely believes me and isn't trying to accuse me he just can't stop his unconscious mind.

How do I help this? What causes it? Any thoughts at all?