it sounds like he might have something wrong with his swim bladder. ive had this happen to my fish when thier diet isnt varied. as odd as it sounds, theyll get backed up and the constipatio will affect the operation of thier swim bladder. has the fish been "regular"? maybe try swithing it to frozen shrimp for a few days.

also, have you done any water changes in the tank recently? if you havn't, maybe its being affected badly by rising levels of ammonia. in that case, it woudl be advantagious to do a Partial water change.

good luck, do tell us how the fish makes out!

Is there enough oxygen exchange throughout the tank? Most of the time just the outflow of the filter provides enough oxygen but in this case, especially with goldfish, it's a good idea to have some sort of extra airator like an air stone of some sort in the tank.

Also like kaiaphas said, it sounds like swim bladder. It happens a lot with goldfish especially if you are feeding it flakes or floating food because when they go to ingest the flakes they ingest air along with the food causing swim bladder. Try feeding him sinking food pellets instead and in the meantime try giving it a few raw or dethawded peas to help allieviate the swim bladder and he should get better soon ^^

Keep an eye on the ammonia levels as well. Very important.

i would like to add that these gold fish around 2-4 years tend to develop swim blatter problems. I have heard from my fish speicalist that a owner should be perpard to put down humanly thier gold fish. so.. jsut throwing that out there.. i dunno how long you have had your fish, but i suspest after 2 years or so.. i would be watching out for this.

Nope, I've only had him a few months.