I have a 10 yo gelding with the same issue. He respects me, and knows where I stand though. I've always watched what my horses do to each other to gain their "titles" and I basically act the way our dominant mare does. I'll protect my herd like she does, I get rough when I have to, and a do a join up every time I mess with any of my horses. My horses don't get away with ANYTHING that's shows me disrespect, and i've had to learn it the hard way. I found that just like dog training, you must communicate in a way they understand, the way they do it amongst each other. And i'm most definitely not telling you to go in the pasture and wave your arms around chasing your horses. You still need to be careful because he may challenge you. Go in there with a lunge type whip and maybe a plastic bag. You need to go in the pasture and claim your dominance there, such as they do. I learned that if you get the "lower on the totem pole" horses to know that you ARE above them and their other leader, its not as difficult to convince the herd leader of this too. Once the respect in the pasture is there, begin with a round work, and successfully join up. I work my horses HARD till they realize that if you stand behind me, your life is MUCH easier. Hope this helps! I was in the EXACT same situation!