I don't have experience with what you are discussing but do wonder what he is fed? I had a german shepherd who had an autoimmune disease which could not be fixed but I did manage to "buy" him an extra two years of quality life by feeding him a "BARF" diet. I went to every expert who all told me that I could only make him confortable and at that stage he was constantly on antibiotics and cortisone injections
I did some major reasearch after my vet handed me the book "give your dog a bone". It changed my thinking and I went all out and followed the Barf diet for the next two years. He was able to come off the drugs and had a much more vital life than he had before.
I do know how difficult it is to watch a loved dog (your best friend really) suffering and feeling like there is nothing you can do. Please google the "BARF" diet which stands for bones and raw food and maybe you can make some changes to halt or slow what is going on and give him a few more quality years.
I know Josepha feeds her cats a similar diet and do believe that just like humans if you can get good nutrition and what the body needs in enough quantities it has a better chance of healing and being able to fight any breakdowns in the system.
I hope he feels better soon