My son has struggled with nightmares, and what we've found is a direct relationship between sugar before bed (about two hours seems to be the cutoff) and nightmares. It also happens when he has to pee at night.
Beyond that easy fix, you can consider hypnotherapy. It's not as weird as it sounds, and it's not outside the philosophy of any religion that I know of. It just deals with the realm of consciousness below our waking minds, and that's where dreams are from. It's fast and could probably be taken care of in one session. Whatever is disturbing his sleep is coming from his subconscious mind (literally below consciousness) and getting into a relaxed state near sleep (which is what hypnotherapy does) can bring out the cause and help him find a solution. If you're interested, get in touch with me. I don't know where you are but I can probably recommend someone to you. It will most likely be faster than psychotherapy.
Good luck. He'll get through it, and hopefully soon!
I wish you well.