add some table salt to his water.. add 5 table spoons ASAP (you said he was in a 50gal right? its 1 table spoon/10 gallons of water.

Turn the tank heater up to 82F & get rid of any bright lights. Just make him relaxed. Leave him alone & hope for the best.. I'd remove the goldfish for now, as he needs to be alone & not picked on.