Hi there.
Sorry if i'm too late!
It looks to me like your fish is in the late stages of Septicaemia. (possibly some kind of Viral hemorrhagic septicemia?)
This is a very sad sight to see, and unfortunately it is most likely going to die.
There isn't much you can do at this stage, I would imagine anti-biotics injected into the fish is needed, but I wouldn't hold out hope for that either.
I really am all for trying to give fish a fighting chance instead of euphanasia, but in my honest opinion, the kindest thing you could do right now is to put this fish out of it's misery.
It is at a late stage, and it IS suffering. This will likely get worse too.
Also please take into consideration, that this is an EXTREMELY infectious disease, and will spread fish to fish very easily.
Good luck!