Sounds like it has some sort of swim bladder disorder. Without knowing anything about your system size, other inhabitants, water quality, or anything else, it is hard to say what to do without more info. I can say, if it were my system and I knew everything else was good, I would isolate the fish in a 10g QT and treat with a broad spectrum antibiotic like Seachem's KanaPlex or SulfaPlex. KanaPlex has worked well for me when I had a fish that was suffering from one thing or another. The hard diagnosis about swim bladder issues is identifying what caused them in the first place. If it was due to poor collecting, it will usually correct itself over time as long as the fish is eating. If it is due to a bacterial infection, then an antibiotic is the best choice. But then why did he get a bacterial infection. Was it poor water quality? Stress? Fighting? Injury? If it will eat medicated food, even better. If it won't eat, then treat the water with the aforementioned medication. A QT is a must in this situation because you don't want to dose your whole tank with an antibiotic if you don't have to. Also, medication can be quickly removed from the water with carbon in a 10g system in case one isn't working and you need to switch to a different med.