swim bladder or digestive problem (bloating pushes on swim bladder and causes problems).
Does he appear to be bloated? If so, stop feeding. Boil a pea in water until the shell comes of. Squish it, and drop it into the tank. The fiber should clean out the digestive tract.
If he is not bloated. Give it a day and see if the condition changes. If nothing has changed treat with an internal bacterial med. Maracyn 1 and 2 in conjunction should knock it out.
Certainly a swim bladder issue tho.
edit: what sort of tank is he in? (size, temp, etc)