yeah, no chance of it growing back. This is nasty but now you just have a "sweetheart jack" lol. Hope he makes it, I have this stuff called "bio-bandage" that's supposed to help for wounds. You apply it directly to the wound with the fish out of the water. It's mostly metylene blue. Maybe something like that might help?
I think infection/fungus is your main concern here if he's still swimming.
Good luck jacky! you will be in our thoughts and prayers