You should never use Selsun Blue on dogs.. Except on rare conditions.. Tar, Coal and sulphur based shampoos are designed to remove skin oils.
Get yourself a bottle of
Theraneem
Also add some
Grizzly Salmon Oil to his diet, the Essential Fatty Acids should help. You also might want to add some digestive enzymes (I use
Great Life Enzymes Pro), it helps a lot with flaky skin.