Hey there.
Can you take photos and post? What are you seeing specifically?
Is he in a tank by himself? a bowl?
IF you are 100% sure he is suffering from fin rot then I would recommend a few "over-the-counter" items such as betta "Revive", Melafix (try to find non-pond variety), and aquarium salt.
Be careful though as fin rot can be confused with many other illnesses (which is why I was asking for photos). Also if he has other "tank mates" then you more-than-likely would be better off separating him in a smaller bowl and dosing him a few hours each day.
Hope he gets better.