When I first read the post, I was assuming that you were the one promoted to herd leader! This, I think, is what we need to work on.
First, we will describe the herd leader. Many people see horses assuming the lead rold as the strongest horse on a power trip. However, it is quite the opposite. A horse does not want to be herd leader, it is actually a very stressful position, but a horse will assume that role if there is not anyone more adept to leading and protecting the herd. An unstable herd leader will affect the energy and function of the whole herd. A good herd leader is not an aggressive horse, however, they are assertive, calm, adaptable, and alert. They do demand proper manners in the herd and will use force to sustain that.
Now, we will move on to your position in the herd. I see many people think that working the horse consists of acutally doing ground work or riding. When you are working with herd dynamics, your work begins when you enter the pasture. You become the herd leader.
Here is an example. A friend of mine had two horses, a mare and her daughter. The mare is a real pushover, as is my friend, the filly took the lead horse role. Very dangerous as the filly did not have the experience to be the lead horse (she is 3). She was never taught proper manners by her mother or my friend and was basically just a loose cannon, similar to how people have to move up to management, this filly just took the position with no prior experience. She became very invasive of space, nervous, anxious, pushy, and even trampled my friend several times. I could see it was becoming an issue, so I offered to let her come to my barn and sent one of my old lesson horses to be a companion for my friends mare. When the filly was introduced to my herd, the other horses could have cared less. However, when the filly was rude, pushy, and anxious, my herd came alive and taught her manners. Could I have done it? Sure, but I took the easy way out and let my herd do it for me. My friend was busy for a few weeks, and then came to see the filly. She couldn't believe it. The filly was quiet, polite, stood for grooming and rinsing, her hair had all grown back in her bald spots, her coat was nicer, she was affectionate and willing, all of the things that a horse should be. I had done minimal work with her. However, just by understanding boundaries and having the weight taken off her shoulders of being the one in charge, she became a happy filly. Her confidence improved and her buddy sour issues went away on their own, just by being in the right frame of mind.
I have a solid herd and an incredible lead horse (who is of course under me in the pecking order). Your horse has assumed the lead horse position without the necessary leadership skills, similar to the filly. You can easily become that lead horse, but it doesn't just mean doing groundwork. Just become one of the herd. The herd does not accept poor behavior. Everytime that you interact with that horse, you are training him in some way. Its not just when you take him aside to do groundwork, its all the time. Every time you lead him, feed him, stand in the pasture with him, you are either above him or below him in the pecking order. His behavior will improve as he finds stability in your leadership.
As mentioned with my friend being run over, this can escalate into a dangerous situation. So now, you take that job promotion to herd leader.