the main thing he needs is some warm, clean water, along with some aquarium salt.
since you already have him in a QT tank,
- if you have a gallon milk jug or water jug, empty it and rinse it, then add water, and some dechlorinator (API stress coat works well, since it helps heal fins), then 1 tsp of AQ salt per gallon. then, pour into tank.
- do daily 100% water changes with the AQ salt, for a maximum of ten days. any longer and you risk harming your betta. you can continue with the API stress coat from there.
i hope he gets well. my betta is now healing from fin rot, and he's doing great. (: