You may have to really escalate your cue then; remember, pretty much nothing you will do will actually injure your horse, so if you have to use a crop and spur, to shake his attention, then do it...that is a much better alternative then him escalating HIS attitude to where he not just stops and pins ears. If you have to, have a trainer, or another adult get on him and give him some corrections, then have them show you what to do, and apply it yourself. When you're riding, is not the time for a horse to act all territorial...it's extremely dangerous!
I used to ride with a young lady who had a horse that was like yours, and one day while she was leading him, he stopped, put it in reverse and laid into my horse with both hind feet...Fortunately neither of us was really hurt (he caught my foot, and my horse's chest) but I was able to put it in reverse on my own horse, so we got out of there fairly quickly. So definitely try to keep his behavior from escalating...it won't be pretty if he attacks someone else's horse/rider, and they decide to pursue legal action.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."