If it happened at a definitive point (as in overnight) and he wasnt like this prior, its likely the result of an injury. I have seen goldies like this before but it was due to a genetic defect in their peduncle that breeders would cull & its something that would gradually get worse with age. I honestly dont think theres much that you can do- if he seems happy & has a good appetite, just let him be. Hopefully, it will heal with time. If it reaches a point thats its quanity vs quality of life, then its a decision you will have to make, unfortunately. An aquatic vet may be able to offer some sort intervention if you would like to seek this route.
