I would keep doing water changes. Sounds like this tank went through what is called old tank syndrome. Basically, it means the cycle has crashed. So the good bacteria has to re establish.
The best thing you can do is do not feed. He can go weeks without eating unless he is already very thin. I would highly suggest a large water change like in the range of 80% or more. the max you can do. Make sure to use a good conditioner like prime to treat the tank. A little aquarium salt and you should be good. Keep doing daily water changed after that (around 50% or more) and see if that doesn't help.
I would give it a few days at most before putting him down.
Stop beating yourself up and try and do everything you can to keep that water in tip top shape. Also, don't run the lights more than 6 hours a day or so. or maybe not at all if the room is lit. let him relax.
Adding an air stone couldn't hurt either. drop temps a bit is not a bad idea either.