Only thing I would suggest, yet by already treating, may make the test off. Is to have a deep tissue scraping done and find out what the heck this is. How frustrating for sure. If is was mites they would be killed off long ago. So now we most likely rule that out, unless he is immuned to all mite treatments. I agree, sore mouth seems wrong. He would have it on his mouth by now. Use a black light to see if it is ringworm. If it isn't. I would try coconut oil as I mentioned prior. It has healing and soothing properties.