Swimbladder is when the fish cant control how it swims
You will notice that he is either a bottom sitter or a floater. Bottom sitting fish just waddle along the substrate and when they try to swim, sink back down again. This is due to the air bladder not able to fill with air for some reason.
When they float its because they cant get rid of air so they roll about on the surface and cant swim down without floating back up again.
Another reason for all this is too much gas in the intestines. The commonest cause is flake food because in coldwater it takes longer for the food to go through the intestine and gives off more gas than usual making the fish floaty. To cure that, dont feed flake food again. Fast the fish for 3 days and then feed it boiled squashed peas for another 3 days and then see if that helps at all. Raising the temperature to 18-20c will help food go through the body quicker.
If that doesnt work then it could be the bladder has been damaged in some way, and something like a tumour is growing pushing it out of shape. Bacterial or viral infections can affect the swim bladder and feeding medicated food may help that.
Switch to a food where the fish doesnt have to reach out the water to get it and dont feed flake unless the tank is warmer and the flake has been soaked first.
You can get sinking granular food for goldies.