Try cleaning it and apply either neem oil or cold pressed coconut oil. Small drop and massage it in. Helps fur grow back. Mine has the same scab looking stuff under the chin.
Hi, there could be many reasons for this from food allegies, fleas, mites or stress. Best is a diagnosis from vet.
As he has bite marks I suspect fleas as there could still be some around the house. You also need to "deflea" the house as well as the cat. My first stray cat had fur mites and had...
