well you have caught the fin rot early enough that you could probably beat it with good water changes, aquarium salt, and stress coat. Bettafix is unreliable from what I've heard/read. I refuse to use it myself.
Looks to me like you've got a tail-om-nommer (tailbiter). If you focus on keeping the water super duper clean, the fins should grow back no problem (provided he stops biting). My tailbiter will trim his fins down every so many weeks when they start to bother him. It's a cycle. Look for clear edges forming as a sign that he's healing. Check the filter current and make sure it's not making swimming difficult for him.