Oh gosh. That's not good, but he's still alive so there's some hope. The fear here is that he might have suffered too much damage for him to come back from. There's a fear of not eating after this and starving to death and also infection. As naiad mentioned, see if he's eating at all. If he is, make sure to feed him vitamins:
- Soak fish food in vitamin supplements such as Selcon, Zoecon and Vita-chem or even Omega 3 & 6 fish oil. This will boost a fish’s natural immune system and is particularly useful for clearing viruses such as Lymphocystis.
You can also give him an antibiotic soaked in his fish food like kanaplex. You'd need to use a binder also such as Seachem's Focus. This will help fight infection of the wound. I mention feeding him the antibiotics because they are all in copper at the moment so it seems the best way to help him.