Rub some pig oil into his skin, veggie or baby oil will do if you don't have pig oil. Wait 24 hours and give him a wash and brush with a stiff scrubbing brush - head and shoulders shampoo works great

Continue to give him an oily massage once a week for a few weeks and he should improve.
Having said all that, if he's not been wormed lately (and an injectable is the best sure fire way to do it) you might be fighting a losing battle. If you're not sure of his worm status, either do a fecal egg count and treat or (and what I'd do) jab them all just to be on the safe side.
HTH