that's a nasty cut! i don't think in will grow back,sorry. my betta had part of his tail ripped... it never grew back. i suggest going to the nearest pet store and ask for a healing medecation. put your fish in another tank and give him the medication as the instruction tells you. then put hin back in his original tank and put some bacteria in the water. im so sorry for the fish, he even looks sad.
good luck!
