when my bettas do some tail biting, i rarely even treat it. just keep up with water changes, add or remove some decor, move things around. more often than not, that nips it in the bud. only one it hasn't is my HMDT, Twitch. DESTROYED his tails. one's almost gone. 8I no reason to it at all. he's in the living room now, beside my rescue female, Fritzy, where he can get more interactions with humans. hopefully, that helps him. :B
your boy, is beautiful. hopefully, when he regrows his tail, it'll be as beautiful as before.