Have you asked your vet about this?
You could also go to the pet store and see what they have? Often small pet stores can give some advice. There is a holistic type pet store in my area that I would go if I had similar issues because their staff is knowledgeable. There may be some food you could give max such as avoderm which is good for skin/coats. Although I'm sure there is something you could put on his coat, not sure if that is wise due to the fact that cats clean themselves.
Oh and I found this article. It basically says the same thing I did:
http://cats.lovetoknow.com/Dry_Skin_Cat