what flake food are you feeding him and what kind of goldfish is he?do not feed him too much as this will just make things worse. raise the temperature to about 27 maybe 28 degrees add a little bit off salt to the water i would use interpets swimbladder treatment No 13
ps: its mainly about diet but it can be the water do lots of tests such as ammonia, nitrite, nitrate, ph etc unfortunately if one fish has it there is a chance the other fish might get it as they are in the same water and on the same diet i hope this helps