That sounds pretty bad. I am going to be surprised if he is going to make it. What other fish are in there with him?
They should be able to recover from a fin nip unless it gets infected so I would put him in a quarantine tank and treat him for any kind of bacterial infection that might have resulted from the bite wound.
I like Seachem products and tend to use their stressguard product sort of as a preventative when stuff like this happens.
Paraguard would be an option once the problem has already started.
Good luck. Hopefully he'll beat the odds