For fin rot, I absolutely would stick with regular 100% water changes. I've had hundreds of bettas in my care and honestly.. the only thing that took care of fin rot for my fishes were regular WCs & a bit of aquarium salt. I tried Melafix (bettafix is the diluted "betta" version) and didn't really notice a difference.

If it was stress related, you'd notice bitten fins instead of rotting fins. If that's the case, then I would find another way to do WCs (but clean water is still crucial to healing fins).. in stress related cases, I syphon the water out.

About his behavior though.. most bettas are fairly old by the time they get to pet stores. He has to be 2+ now so he may just be showing his age.