That's a really tough call, but if the infections are getting worse and he is obviously not eating then the chances of him making a recovery are slim to none. I would pull the plug.
If you have a spare tank (10gal or 20gal), move him in there and treat him with Metro+, penicillin, salt, high temp (84 range), good filtration (no carbon) and plenty of oxygen. If not, pull the plug before he takes the whole tank down...