My 19yo Son Has Been Having Some Back Discomfort And

My 19yo son has been having some back discomfort and thought he had hurt his tail bone. He has just shown me the top of his bum crack and it's red inflamed and the size of a 50 cent coin. No broken skin or bite marks but looks very sore.
Never seen anything like it before and it is causing him discomfort standing sitting or laying. I plan on getting him to seek medical advice tomorrow but was trying to identify what it might be.