Welcome to MFK!!!
He can "unstick" himself. Suspend the barrel at the top of the tank with about 80% of the barrel out of the water (or leaving enough water in the barrel to cover the cat). Have the largest opening pointing down in the water. Turn off the lights and leave him alone. You could put some sinking food in the tank to help coax him out. He'll get himself out of the barrel in his own time. Trying to force him only makes him puff up and lock his fins out.