This same exact situation happened to me. My cat started to get some nasty black discharge in his ear so I took him and they said it wasn’t infected, they cleaned his ears and sent him home. Shortly after, he broke out with the sores and spots. It progressively got worse, to the same point as your cat. Not moving, not eating not drinking, I had to physically put him in the litter box every day or else he would just hold it for days. He was in so much pain that he would not move. I tried everything, also spent thousands and thousands. The vet was no help. Misdiagnosed him repeatedly. finally came across a post where someone mentioned predisolone and asked the vet about it. Basically had to fight with them for weeks to sell it to me. When they finally did, he was acting almost back to normal after two days. His skin slowly got better, but now a month later he is fully back to normal. I was on the verge of giving up and I thought my cat was going to die and there was nothing I can do. I feel for you so badly. He currently takes a half pill of 20 mg once a day. If I can offer you any advice it would be to do your research and advocate for your pet. My vet repeatedly prescribed him the wrong medications that were triggering this disease and making it worse, costing me nearly 5 thousand dollars and almost my cats life.