Hi vier. I'm sorry your guy doesn't look so good. It looks like he has septicemia, an infection in the bloodstream. Also, do you happen to know what your nitrites, nitrates, and pH levels are? High levels of those can be just as bad as high levels of ammonia and high nitrate levels especially are known to cause shredded fins. It's so easy for us to forget to test those levels (I know I'm guilty of this) and only focus on ammonia levels.
For now, can you switch him from AQ salt to epsom salt? The epsom salts will help to pull fluids from his body, enabling him to fight back against the infection going on. The dosage you want is 2 tsps per gallon of the epsom salts.
Ideally, I'd like to keep him in the epsom salts for a week with daily changes and give him a chance to fight back on his own, because too much medicating can be just as bad as not enough. If by the end of the week, it looks like the epsoms are working, then we'll continue for another week.
Also, do you think you can add tannins into the water? You can do this by using Indian Almond Leaves (which, regrettably are hard to find and almost always have to be ordered online) or by crumbling clean, dried oak leaves into the water until it turns dark tea-colored. Tannins have some medicinal properties as well as the benefit of replicating a betta's natural habitat (dark colored waters).
But if he starts to take a turn for the worse at any time, we'll have to go straight to meds. Are you allergic to sulfa drugs?