Any chance it could be environmental allergens from walking outside? I've been washing Tyler's feet when he comes in because of some of the trees shedding here. Otherwise I'm sure others are more knowledgable than me.
The thing is I know what allergies are having had a dog full of them. This is no allergy. He is not chewing on his paw and not even chewing on the leg. He starts on top or bottom of the leg and gives it little bites going from to to bottom or bottom to top. I was wondering if he could have arthritis on that leg, but than he walks fine on it.