If you don't hear from OFL, and If you have a Kritter Keeper, I would start treatment in one, with 1 tsp per gallon of clean, warm, dechlorinated water, that would be a good start. You can use your main tank, if need be, but you will have to do daily water changes.
Poor fishie :( I really hope it's not dropsy. Both my fish in my divided tank got a little bloated at the same time.... I started panicing because I though they both had bacterial infections, or something similar. I fed them both a tiny piece of pea, which I know alot of people do not agree with, but it was the only thing I had on hand. And did not feed for a day after... It turns out they had eaten the algae wafer pieces I had put in for the snails... little piggies. Anyway, I hope OFL gets back to you soon. Best wishes for the fishie <3
I'm sorry your boy isn't feeling so well. What size is your tank? What is the temperature? Is it heated? Filtered? How often do you perform water changes? How often and what do you feed? If you look at him from above, do his scales poke out and make him look like a pinecone?
This looks like fluid build-up. I would start with a 100% water change. Either change his current tank 100% or use a hospital tank. Treat him with EPSOM Salt at the rate of 3tsp/gal for serious swelling. You can find Epsom salt at your local pharmacy (Walmart, CVS) for around $3. Use the unscented variety. Do daily 100% water changes, keep the tank around 82-85*F (no warmer). If he does not improve in the next few days you may need to consider a medication like API Tetracycline or Maracyn I & II. Good luck & keep us posted!