Did you recently bring him indoors or did this happen outside? I would recommend contacting your vet and ask if you could send them this pic.
In the meantime, to help with healing and reducing irritation, try this home remedy, courtesy of Mamanyt1953
: "Buy a box of chamomile tea bags in the coffee/tea aisle of the grocery store. Get PLAIN chamomile, no blends, which will be clearly marked. I recommend the commercial bags because they are all 100% German chamomile, which is medicinally active and safe for cats. The English variety which is popular in gardens is pretty useless medicinally, and TOXIC to cats...brew a cup of the tea, and chill it in the fridge (it will keep in there for about 3 days)...For external use, simply dip a cotton ball or clean, soft cloth in the tea, and swab it on the area. The tea has both soothing and healing properties, and can be dabbed on the skin multiple times a day. It won't cure what is causing the health issue, but it should help to make him feel better.