He isn't over fed chances are either he is suffering from swim bladder, bloat or the start of dropsy.
For swimbladder and bloat, only epsom salt never table, aquarium or other salt in the tank. 1 tesp per gallon. Thes are electrolites not salts and won't add to the bloat.
If he is doing nothing but sitting and his scales are lifting, it is the start of dropsy. Sorry anti bacterial is the only thing at this point that may help.