I would suggest washing it off well with a mild dog shampoo and then cleaning it off with Betadine iodine solution or 3% hydrogen peroxide. You can then use Neosporin or Triple Antibiotic ointment on the sore areas. None of that will hurt him if he licks it.
But I'd plan a vet visit for him in case this is a problem with mange mites, ringworm infection or a skin infection that will need oral antibiotics to clear up.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics