We, too, had something similar to this with a Miniature-Schnauzer that we had years ago. She was allergic to grass
!!...... We kept her out of it as much as we could for awhile and used oatmeal shampoo and also an oatmeal based conditioning spray. In no time her skin was back to normal. (and sure enough, if she was outside in the yard too long..... it would come back)
Good Luck in finding the cause of his skin problem..... have you tried an oatmeal based shampoo? It does help soothe the skin, too!