Take in a stool sample as well to have him checked for tapeworm (possible because he has had fleas) as well as any other worms. Whether this is related to his condition or not it's a good time to do it since he is going anyway (if it hasn't already been done). Have the skin scraping done to check for mange (demodex or sarcoptic) and have him checked for fleas again. If all of that comes back negative I would suspect a food allergy or a possible allergy to something he sits in since it located around his hind end, possibly grass, weeds, or something of that nature. He also may have had a reaction to the frontline and it's cropping up in those areas. Good luck and let us know how it goes. He really does need to see the vet for this though, so you and he can rest easier.