Ugh. Sorry that your guy is ill! Very strange looking - I doubt that it’s one of our typical poultry diseases like scaly leg mites... It would still be worth posting on the Diseases forum in case someone has seen this before. To me, it kind of looks like a burn or infection. I’d put infection high on my list. Infection could be of the skin and soft tissues, or of the underlying bone. Is a vet possible? They could take an X-ray and see if the bone is infected.
Treatment is going to be a challenge if he’s on the wild side... You need a way to catch him daily, like a sick cage. I think daily Epsom salts soaks, maybe with a little added betadine, wouldn’t hurt and might help draw out infection. Maybe you could put the warm Epsom salt soak in a 5 gal bucket, with the level just above the height of the injury. You will probably need something over the bucket to keep him inside. If a vet was involved, they could send home antibiotics.