Have you checked your other chickens yet for the start of anything similar? What does his feet look like? I could be VERY far off, but in all the pictures I saw what appeared to be Foot Parasite, though that usually goes after the webbing in a chicken's foot. It simply barrows into the foot webbing up into the leg. Perhaps this is what it is? If it is, it's spreadable, so treat it as a disease and try to prevent spread. Again. I could be very far off! Hope he heals.