Betta Fish: I’m so worried about my boy :(

My boy Sam has what we’re pretty sure is Popeye, THIS IS A HOSPITAL TANK. We’re currently treating the water with Melafix according to instructions, but he just seems so sad 🙁 all he wants to do is sit on the bottom of the tank. He perks up for food but nothing else. I’d be so upset if he dies in this tiny empty tank, he loves to swim and explore the plants. He usually gets so excited when I walk by his tank. I don’t want him to die ofc so I’m trying my best to treat him, but how do I know if the treatment doesn’t take, is it possible to move him back into his ten gallon tank if he’s untreatable? Sorry the photos aren’t great, he heard me walk to the tank and got excited 🙁 any advice is helpful