You'd probably need to treat for a month or more, maybe months, IMO if you try the apple cider vinegar, and you'd need to keep him on it long term so the gout crystals wouldn't grow again, I wouldn't stop too soon.
Also, for treatment of scaly leg mites, consider using Nu-Stock, which is a...
Man, I don't know what to tell you. Possible cancer? Could be a systemic infection. Have you tried giving him antibiotics? If so what kind and what dosage?
@azygous or @Eggcessive any thoughts?