Since you mention in another thread that you're feeding him Ol' Roy dog food, I would suspect the problem is his food. And accordingly would suggest that you try upgrading him to something better.
Since dry skin is often related to deficiencies in things like Omega Essential Fatty Acids, you might also try adding supplements high in those to his diet. That can be in the form of oily fish such as sardines, pilchards, mackeral (even canned is OK, preferably in oil or water, not brine). Or you can add fish oil capsules to his food once or twice a week. Other useful sources are flaxseed oil, sunflower oil or olive oil.