Have you considered moving him into a smaller tank? It takes way less time to clean. Trust me, if I can find time to clean my 2.5 g each day you should be able to. I just got done fighting fin rot with my guy and aquarium salt and clean water cleared him right up.
ETA: If an animal is still eating, it's not ready to go yet. Animals stop eating when they are ready.
I am in no way an expert, but reading off the sticky Betta Fish Diseases Part 1 in Diseases and Emergency (posted by I believe DarkMoon17), it says "API Tetracycline, API Fungus Cure, API Triple Sulfa, OR API Erythromycin".
I can try, and if it doesn't work?
I will keep you guys updated when I start the AQ salt treatment.
Will the betta on the other side of the divider be okay? He
Is very healthy, but I don't know how he will react to the AQ salt.
I definitely don't think you're even close to having to make that decision. I wouldn't consider it until the betta is lying gasping on the bottom of the tank, totally unable to swim, and/or pineconing.
As MattsBetta's has said, keep up with water changes, keep the IAL, use AQ salt, maybe try some StressCoat. High protein food and water in the 82-84 range will likely help too.
I will raise the temperature, get some AQ salt ,And I don't have IAL leaves. But I will try my best. I am hoping to clean the tank on Friday, so I will start the AQ salt treatment on Thursday.