I'm not sure, but it certainly is something he should see a doctor about ASAP. It might be a bacterial infection, of the kind that could appear anywhere on the body. I suggest that he apply bacitracin or neosporin ointment until he gets someone to see it. If he's in the habit of picking or scratching it, he should stop. It's worth the investment of a visit to check this out.
Take care.
Dr. Rockoff