it's fin will grow back naturally, with it been a young fish it should grow back faster, but I think with the fin been that badly damaged over half missing I would consider cutting the other half off so it grows back even.
Most it's tail is missing anyway so it would not make much of a difference. I had a arowana with damaged front fins & a bite had been taken out of its tail but it grew back within 4-6months .
The main thing is crystal clear water & good food