If not a ring Worm I would consider that the pup may have hit his back on something and knocked the hair off. The reason I say this is because I recently had a male do this to himself. Long story short.. I have the sled dog type A-frame dog shelters, I moved another male to a chain spot next to his and he got excited and started doing his thing. Next thing you know he had smoked the side of his shelter between his shoulder and ribs. Looked just like you shaved a circular spot with a razor.