Warm, clean water is what stimulates fin regrowth along with the Stress Coat conditioner you're adding. How much aquarium salt are you using? Generally 1 teaspoon per gallon is fine but do not go over 14 days using AQ salt. It can actually harm their liver and kidney's when tried to use as therapy.
So instead of using salt, just keep the tank clean. I suggest doing two gravel vacuums a week (you can either do two 25% or two 50%) either way is fine as long as you're keeping unwanted parasites out of the tank.
Keep the temperature around 78 degree's if you can, and just add in Stress Coat conditioner each time you add new water. (No matter which conditioner you use, always, always! use conditioner before putting new water into the tank. Chlorine and heavy metals are harmful to fish)
Also a high quality diet will help stimulate growth, if you can find mosquito larvae around, give them a rinse in some conditioned water and then you can feed a couple to your Betta's with a dropper of some sort!
Or you can get higher quality pellets like New Life Spectrum or Omega One, those will certainly help. And remember to scoop out any uneaten food, that will rot and make it easier for bacteria to get into his fins!
I hope this helps and he heals up soon! With good care, he should start to stitch it together by the end of the week