I would go out and get a smaller tank and put him in there. prepare the water in the new tank, pop some water from teh old tank in for him. pop him a plastic bag with water from teh old tank and float him on top of the water in teh new tank, then after about 10-15 minutes, slowly and gently pour him into the new tank. this way if he does die, he does so in a separate place to all the other fish. also if he has an illness this could prevent spreading ITMS.