What Is The Treatment For Painful Red Excoriated Area Around The Anus?

My 7 year old son has come to me complainig of a painfull red excoriated looking area around his anus. It looks like it is the area that doesn't get any air, no bleeding, no constipation, etc. No pinworms, and it doesn't itch. He rides his bike all the time and sweats a lot playing baseball. Yeast? Heat rash? What to do?