I just noticed a quarter sized swollen spot on my 9-10 week old puppys hind led no hair loss not itchy but it has a greenish puss/scabby substance on a small fraction of it what could this be?

Hello I have an 8 week old male puppy named diesel. He had been dewormed and had his first two sets of shots. I’ve noticed he had a quarter size spot near his tail. It’s crusty and has hair loss. I’m very worried