There are many good ideas here and I myself work with youngsters but a little younger than yours. The first step (I use) to breaking a horse is long reins. It sounds like you have worked with a lounge line (just the single long rope) but long reins can go through the stirrups if you have the saddle on or through rings on girth that goes around the horse (I’m not sure the technical term). Just do exercises both directions and even over SMALL jumps. You can attach them to either a halter or for maximum control (and my personal preference) to your riding bridle without the reins or make sure they will not interfere with your cues. If you are confused by my explanation, you can check out a youtube video or two (may sound bad but with simple long reins it is possible to see how to do it on a video). You may also know about this method and use it but just trying to help. one last thing (you have probably done this too) make sure its not something specific or a certain area only that he does this some OTTB will have had not such a great home and weird things will spook them). Hope something we all have said helps.