Hi,

The goldfish breeder was right, its called swimbladder disorder, the swimbladder is a ballon of air in the fishes body that controls its boyancy in the water and in fancy goldfish its really common for it not to work so well as the fish are bred with much shorter bodies than natural, so all the insides are squished up together.
First try starving him for a few days (make sure your water is good, ammonia and nitrite 0ppm, if not do a water change) then feed him shelled peas and see if that helps, if it doesnt help, starve for another few days and feed peas again. If doing that a few times doesnt help, it may be a bacterial infection, you can buy medications for this at a fish store. If that doesnt help it may be incurable without surgery, but this is expensive, risky and stressful on the fish, many fish live happily upside down if this is that case.