In a horses mind, a top priority is to evade pressure. For most horses, that means bracing against it, or pushing against it, or fleeing it, or fighting it. You are seeing that in your boy with the head tossing. I am willing to bet he is doing it in a dozen other ways too, that are more subtle or that just don't cause you too much problem, so you don't recognize it or worry about it much. That is normal in a beginner. Don't beat yourself up. Instead, look for all the other ways he is bracing or avoiding/evading. You have to go to the beginning and teach this boy a different way to think. He needs to learn that pressure can also be made to go away by going soft and yielding to it. This is counter to the way his brain is wired, so you will have to teach him this and condition him for it. This will take time and work, because he is pretty far along in his training to have to learn a whole new way of thinking. Furthermore, yielding has to become what he tries FIRST. He is going to have to completely rewire his brain. If he has had some success with bracing/tossing/fighting, this is going to take some consistent effort and good timing.
One of the things I do with my very young horses--the ones I am not yet starting the groundwork on for riding, is to take my right hand and put it high on their neck just back of the poll. Then take my left hand and put it over their nose about 3 inches above their nostrils. Put gentle downward pressure on both hands. When you feel the horse start to brace against you, continue with the gentle pressure and start rocking your hands horizontally back and forth. Most horses will dip downward just a tiny bit with their heads when the pressure changes from gentle steady downward pressure to the gentle downward pressure with rocking movement. The very second that you feel him dip his head down? Totally release him with both hands and tell him he is a good boy. He just took his first step in evading pressure by GIVING to it. Do this a couple more times just to establish in his mind that giving to the pressure WORKED! It made the pressure go away! In his mind, this is now another tool he has besides bracing/tossing/fighting. Do this regularly while you are establishing it. Find ways for yielding to pressure to give him what he really wants. Release. Get his mind working for you. You can do this in very young foals even. That is what it all comes down to. You don't want blind or fearful compliance. You want a horse who has been taught to think like a partner. That is the basis for conditioning for response. You have to set the foundation in his mind that he needs to find a way to yield to your pressure for the pressure to go away. When it is done well, it is amazing how little pressure it actually takes for a horse to respond. It is a beautiful thing--both art and science.
I have an 8 year old Arab/Quarter gelding who is a very sensitive soul. He is pretty athletic and pretty reactive by nature. When he was very young, on one of our first rides out of the arena the first or maybe early in the second year he was under saddle, we had an incident where we were out riding in the late evening by the side of the road. Apparently, a mother deer had bedded one of her fawns down in our flight path and we didn't see it in the twilight. As Dakota stepped over this fawn, the little guy finally stood up bleating. Dakota (my gelding) nearly had a heart attack. He gathered himself, spun 180 degrees, and then just stopped. I felt him quiver, he was tight as a drum--take a deep breath--and then he dropped his head down level with his chest and willed himself to go totally soft. It was his language for "help me--what do you want me to do?" He was yielding to the pressure of being scared by the fawn, and looking to his leader to keep him safe and tell him what to do. It was totally counter to his reactive nature. But he had been taught how to think that way and when the chips were down, he went back to what he was taught. It nearly made me cry. It is a beautiful gift when a horse overrules his nature and yields to his rider and has enough trust to put himself in your hands that way. We planted that seed in his head when he was a yearling and I started with the exercise above. Lots of others as well of course. But we started with that one. That is what I mean when I say "condition for response". It has to be so practiced, and so ingrained, that it is his first line of recourse when he feels pressure of any kind.
