Using aquarium salt for fin rot is fine. But aquarium salt contains sodium, which can cause fluid retention and buoyancy issues.
I would stop the aquarium salt, and switch him to Epsom salt.
Epsom salt has a laxative effect, which should help relieve his symptoms. It also has minor antibacterial properties, which will help the fin rot. (See the previous posts about how to use it.)
You said you lost two other fish. Was this due to an aggressive fin rot? If so, salts (whether aquarium or Epsom) may not provide enough treatment. If your other two fish died from a fast moving fin rot, I would treat this as an infection. Since it's affecting the fins, it's probably a gram negative bacterial issue, so you could try something like API Furan 2 or Jungle Fungus Clear.
Also, it sounds like you're overfeeding him. How many pellets does it mean by feeding him "2x a day as many pellets he eats"? That could be a lot. A betta's stomach is about the size of his eye.
What pellets do you feed him? A lot of bettas become constipated if given pellets with lots of grains (soybeans, corn, wheat). If yours has this, try switching to a higher protein diet. (But it still sounds like you may be overfeeding him.....)