I have no experience with lupus, but Spencer took prednisone once after a seizure and the response was amazing. He went from being listless, almost catatonic, to walking and eating within a day. I have had vets tell me there was little to no hope five times now and every time, meds, force-feeding and TLC made the difference. Sometimes, the will to live, a lot of love and the right meds will help them go on for however long they can.
Sounds like you've had all the tests, but did the vet rule out tick disease? That can cause swollen lymph nodes, lethargy, loss of appetite and other symptoms Flame seems to be having. I hope Flame continues to feel better! (If you have trouble getting food down him, Hill's critical care canned food is like a souffle and goes down easily in a syringe. It's gentle on a queasy stomach, too. That was a lifesaver for me a few months ago.)