Think on that hatching link there are some stories and pictures of chicks with this and what people did. Have you seen them? If it is not real big or is not dragging I would just leave the chick in the incubator alone and leave the humidity at the 70 or so you have it. Do you have the other two chicks out of there, I would take them out. Try to keep him quiet, make sure it is really dark and quiet where the incubator is. If it is a little bigger and he is not running around, you can try to wrap his hind end and keep him quiet in a little jar or cup, I have used those little jam jars before, or have seen pics of people using small glasses or little plastic tubs to set the chicks in.