Personally, if he didn't pick up fairly quickly I would consider euthanasia. At that point of emaciation, his internal organs would most likely be damaged, and he would obviously be suffering because of it.
I would definitely isolate him into a 1 gallon quarantine tank as soon as possible. Since at this point in time he needs to hold onto every little bit of energy that's available to him, I would drop the water level down as far as the heater allows and provide something for him to rest on.
It could be internal parasites or worms, so starting him on some medicated food or something similar could be all he needs to pull through. Then it would be a simple matter of fattening him up on some good quality food and making sure he maintains a healthy weight from then on.
Also it's a fallacy that older animals should be skinny to the point of emaciation like that. It's advisable to keep older animals leaner to avoid excess strain on their bones and joints, but they should never have the appearance of being in the latter stages of starvation.
Sending some healing vibes your way, but if it's passed beyond that point, I hope you have the strength to do what's right.