Personally, I'd stop adding the salt and just give the fish a chance to heal with super clean water and good food.
Salt won't do much at all to help if there's an infection, and adding salt to the water can cause mild stress since bettas are strictly fresh water fish. Salt can act as a mild antiseptic, but I always try to let fish heal on their own first, and it's always worked beautifully for my bettas over the years.
So I would stop adding the salt (or look into an actual medication if his popeye still hasn't resolved) and just keep the water super clean and warm, feed him really good food and observe him for signs that it's worsening.
Sometimes when large chunks missing from the fins can be tail biting (that's what it looked like when my boy was biting) or just an injury, but you've just gotta see how it progresses (or doesn't, hopefully!) to see what's going on.
Here's a link to a discussion of fin loss vs. fin rot that really helps me to identify what's what:
Hope this helps!