Shar pei dry skin advice?

My shar pei has black skin on his chest and under side, he itches alot. He is a happy dog and his coat is immaculate after alot of trial and error with his food. He has specialist food as he has an intolerence to wheat and he has medicated shampoo on prescription. Over all he is in good health, apart from the black skin down his front arms, chest and under side. Are there any oils I should try, I'm thinking about trying an unscented body oil for humans on his dry skin as no creams work. We've tried e45, sudocrem, udder cream the lot. Any advice or shared experience would be gratefully recieved x