My puppy is taking hair off of his hindquarters and will chase his tail ..?

I have noticed my 10 month old puppy has been chasing his tail, I think he is also chewing his butt cheeks so much so there is hair loss and swelling/redness- his tail is also missing a strip of hair where he has gnawed it off. I love my baby and I want to know how to fix this?! Could it be dry skin or is it worms- I've checked his poo but I have yet to see anything come out.. I know he doesn't have fleas because I got rid of them when he was a younger and I check for them monthly.. anyone have any ideas??