Please - if he is not getting professional help already, and plenty of it, he must start TODAY. No matter the cost. Depression in daylight would be tough enough, but these dreams sound like he is going beyond depression. If he truly "starts to believe these dreams are real," he needs some immediate help and possibly medication. Do not let it get to the point that he "self-medicates" with alcohol or drugs, or you will have another huge problem to fight along with his mental health issues. Someone said to ask him if pot had previously helped him sleep -- don't! Why introduce that idea into his head when he's already not thinking straight? He needs therapy and medication, not suggestions about finding some (illegal) temporary relief.
He truly, truly needs to give very serious consideration to dumping this stressful job. If he carries your health insurance -- which is essential for him to get treatment -- you and he need to find out now about whether he can get disability leave. Please talk to his therapist if he has one (if not, why not??) and discuss getting him some leave from this job to get this under control. His job MUST give him disability leave if his doctor orders it, but you may need some form of advocate like someone in the HR department and/or legal help if his workplace is cruddy and punishes people who are sick.
He cannot go on like this. If his sleep is broken up, he will eventually start to hallucinate and have other serious mental health issues -- we cannot go too long without proper sleep. If he actually begins to believe you are cheating on him, he is becoming paranoid and delusional and those conditions are more than "dealing with some depression issues." Once he goes down that road it will be extremely difficult to get him to accept help -- and you may find your marriage disintegrating.
Act now before he gets to a point where he's refusing help because he's so convinced people are out to get him, or you're lying to him,etc. He MUST see a professional. Don't expect to stop these dreams on your own or on advice you get here on Mamapedia.
If he's already seeing a therapist -- is he hiding these dreams from the therapist? Why?