just curious......when you put him in your brother's tank, what kind of fish were/are in there?
other male bettas or what?
good luck on restoring the fins of your little buddy! but in time, yes, they will grow back. just make sure he doesn't have to be put through that again.
seeing as you caught it so soon then yes it will heal and hopefully nicely. One of my boys I rescued from a tank full of black widows. His fins were so tatty and his dorsal fin has a kink in it. His other fins are healing nicely but I think the kink may never go. Never mind, I'm just happy I've got him