Hi,
I recommend cutting or shaving the fur down in the spot. You need to clean the area well with soap and water. You should then rinse the spot with peroxide and water. Make sure you dry it thoroughly and apply a antibiotic ointment to the spot. Cover it with a bandage and keep it dry. The biggest issue is keeping hm from chewing at it as this can cause infection. You may need to get a special collar at your pet store or local vets to prevent him from chewing the spot.
Make sure you clean the area twice daily until it clears up. If you notice signs of infection such as a draining pus, four smell, streaks of red or your dog becomes lethargic a trip to the vet is recommend to stop the infection and prescribe a antibiotic.
I hope he is feeling better soon, let us know how he is doing.
Walter
Breeder
Breeder and pet health consultant. Have spent a number of years with dog rescue, training and health