With any luck it is just an unabsorbed yolk sack, you might want to try putting him in a small cup or plastic container to keep him quiet and give it a chance to absorb, maybe wrap his butt in a damp paper towel (it shows a duck in a cup in the one link as an example) If the other chicks are done hatching keep him in the incubator for now, otherwise put him in the brooder in a warm spot but watch it so he doesn't get cooked by the heat source... with a little luck he may be OK.