Thanks to all for the help and advice. I wish taking him to the vet for a "no holds barred, whatever it takes" treatment was possible, but unfortunately it isn't . The vet would give him dexamethasone and sub-cutaneous fluids. We can do the fluids at home, and TLC will have to cover the rest. This is a special cat, and I will do the best I can for him. He is old enough to have some systemic issues, but we'll cross that bridge when we come to it. For now I'm just trying anything/everything to get him to eat. He is nibbling at things, and drinking water. So for now he is holding his own.
Thanks again, and keep the food advice coming!