Does he usually toss his head some or is the head tossing a completely new behavior? If it is already a common thing that has just gotten worse, then I would venture a guess that it is a behavioral problem. If it is a new thing that just suddenly sprang up, then I would bet it is due to discomfort. It could be the sores on his nose getting touched by the halter hurts, or maybe he has a bug in his ear that is chewing on him. In my experience, those little crusty scabs in the ears are usually caused by flies and gnats going to dinner in there. Those little ear-fly guard things would probably help.