If he is due for flea treatment, I would start with that. I know you say it's not that, and that you haven't seen fleas, but I had the same issue with my dog. I have two dogs and the other one wasn't scratching and I never saw any fleas, although they WERE due to be treated. As it turned out, my vet recommended treating for fleas before doing anything else, and loe and behold - my dog quite biting that spot by his tail and back legs. As it turns out, my vet said that some dogs have more of an allergic reation to flea bites than other dogs do. You can have two dogs, both of which have only a few fleas, and one dog won't scratch and the other one will be miserable because he's more allergic to the bites.