I'm suspecting it's more to do with his brain than his swim bladder. One of my blue platies was a lot like this after I transferred him to a new tank. All the other fish handled the move just fine, even really elderly tetra, but the one year old platy was struggling like that right after the transfer. I suspect he whacked his head or something while trying to evade the net and did some brain damage. An infection in the brain could also move fast.
I think he's dying too and would euthanise to prevent further suffering. My platy struggled for a few days, still trying to eat and everything, but day three I found him on the substrate flipping and curling like one of those kids toy fish that curl up due to the warmth of your hand. Had to euthanise him then. Really sorry dude.
You have some beautiful fish in the background, love that black molly especially.
Usually an internal problem, maybe organ failure caused by a disease. It's hard to say without doing a necropsy (fish autopsy). Generally when a fish can't stay upright and stops eating, it's time to euthanise.