Looks like his swim baldder, I would place him in tank w\o decorations, keep the lights off. If the swim bladder is just swollen it will take some time before it heals and he will pull through, if its torn he will not.
Avoid feeding him for a few days, because if it is swollen it pushes on their digestive track and it will just cause more problems. If after 2 days he is still alive try feeding him like MET/Flake. Because this is a internal issue treating the whole tank with antibiotics really doesnt work.
Good luck