Swimbladder issues are easy to fix most of the time. If you do end up getting him, get him in some clean, warm(78-82*F) water and premix 1 tsp/gal of epsom salt. You have to do 100% changes everyday so keeping him in a smaller container would make it easier. Fast him while he's on the epsom salt for the first day and then offer him food on the second day. That gives the epsom salt enough time to push out whatever is causing the issue with the swimbladder.
You may want to pick up a parasite medication, I suggest Jungle brand, just in case it turns out he has internal parasites.
The most expensive thing would be the parasite medication. Epsom salt is very cheap and can be found in most grocery stores.