Fin rot is the direct result of poor water conditions, To cure finrot in 99% of cases all you need is lots of clean water at the right temperature.
You could also try a medication.
Really? What medication would you suggest?
Can you please answer the following questions.
How big is the tank?
Has it got a filter?
How strong is the current?
Is it cycled?
How often do you change water?
How much water do you change?
Do you vacuum the substrate?
What additives or chemicals do you use? Eg De Chlorinator?
Do you use tap or bottled water? If bottled water please give us the brand.
Do you have a water test kit?
Can you tell us the readings for ammonia, nitrite and nitrate?
When you clean the tank can you tell us exactly what you do.
What do you feed him? how much? how often?
Bettas sometimes bite their fins for various reasons, Eg the current from the filter is too strong, This is not really a problem as long as the water is super clean.
I have 5 Betta females ( Sorority tank ) and there is always at least 1 girl that has a bit of fin damage at any given time but this isnt a problem to me the fin grows back in no time.
I would also remove the plastic Christmas tree and Santa, you have no way of telling if they are aquarium safe or not.
aquarium salt coming tomorrow.
Do not add it to the tank for now,
Regards
Nick.