Questions about the swim bladder in bettas

[AlphaFish]

I first noticed my betta lying on the bottom of the tank 2 days ago. That was about the time when I started feeding more than usual. He also had some trouble swimming to the food on the surface, which I thought was because of his long fins. Today I rebuilt his tank kind of, added substrate and some more plants. Now, despite the filter being on the lowest setting and also facing a wall of the fish tank, he still had some trouble swimming, as if something was pulling or pushing him. He also sits more vertically than he should. Did some research and apparently it's a swim bladder issue, caused by overfeeding (also, it is to note that he has an injury on his side, which I think he got by bumping into a rock. Not much he could have bumped into, since the tank was almost barren before I rebuilt it today, except for some plants, the filter and the heater. This injury was healing until today, my sister, who is the one who gave me this fish offered to take it out. So I went to the bathroom to clean a rock that had some brown algae on it and when I came back the betta had something red on his injury and also some hanging scales. She said she couldn't take his out and also didn't want to stress him. I think she saw that she hurt the fish and gave up. I'm now going to stop talking about this since it already took half of the post so far.) So my questions are:
1. Would fasting him for a few days be enough? That's what I saw on some articles, but I am curios about some more opinions on this.
2. How severe is this? Is there any chance that he won't be able to swim to the surface and drown?

 

[Ghelfaire]

Fasting helps a bit. You can try feeding daphnia which acts as a bit of a laxative for carnivorous fish.
Do you have a betta hammock or and decorations close to the surface he can lay on? My bettas love anubias plants if you wanted to go with something alive

 

[Rose of Sharon]

I know that you didn't ask about this, but thought that I would add something concerning his injury.

If he is the only creature in your tank, then you can add some aquarium salt to help the injury heal. If you want to do this, follow the dosing instructions on the package. If you have any other tank mates, like snails, shrimp, or scaless fish, then do not add aquarium salt.

The thing to remember is that aquarium salt should not be used for more than 2 weeks at a time, and that aquarium salt can only be removed through water changes. So as you do water changes, you would want to keep track of how much you take out, so you would only add the correct amount of aquarium salt that was taken out, and not a full dose.

Also, I would do some small frequent water changes every other day to keep his water pristine so the wound doesn't get infected. Adding Indian almond leaves (catappa leaves) to the tank may help as well. They release tannins which will tint the water brown, but the tannins do provide some antibacterial and antifungal properties, so that's extra added protection.

I hope he gets better soon!!!!