Poor Tommy! It could be from an injury. I would gently clean the area with warm water and antiseptic soap. To keep him from licking at it, you can put an Ecollar on him. You can get one at a pet store or make one at home. Keep the wound clean and dry, and monitor it for signs of infection. If you see redness, swelling, warmth to the touch, or pus, then it is infected and he will need to see the vet for antibiotics to treat it.