Omg, thats sounds horrible and painful for your fish. I've heard of fins growing back(with some help of salt and/or melafix or some other medicine for wounds), but if part of his body has been cut off also.......i dont know. If he does make it through, it will be a long painful process. Definitely keep a close eye on him. He will be vulnerable right now to disease and parasites. But if he's eating, thats a good sign.
I've never owned a betta so maybe someone else could help you answer in more detail...
hope all goes well!