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So it sounds like he has a moist superficial skin infection which is commonly known as a "hot spot."
One of the best things you can do is to take a clippers and clip the hair from around the lesion so air an get to it and dry it out.
Then clean the area with Betadine. That is a brown liquid antibacterial skin cleanser you will find in the first aid aisle of the drug store.
Then take some hydrocortisone cream and neosporin antibacterial cream 50:50. Apply that to the area twice a day for infection and inflammation.
Also start him on benadryl for the skin irritation.
1 mg of benadryl per each pound of dog every 12 hours.
For example a 100 pound dog would take 100 mg of benadryl every 12 hours.
If he scratches at it then it will not heal.
You can buy a cone or inner tube shaped e-collar at Petsmart to prevent him from scratching.
Does that help? Do you have any additional questions for me?