Has the BM talked to you previously and told you your horse was an "issue" and allowed you time to "fix" it? I am just curious sine just the other day on here we had someone post who was on the other side of the story, of sorts...there is a horse at her barn who is a danger to the other owners and their horses. THe advise here on the forum was to get the BM to tell the notify the owner and fix the dangerous horse, or give the owner the ultimatum. THis sounds rather similar to me.
My other question would be-if you are a minor, are you comfortable telling the adult (BO) hands off your horse? If not, you may want to have your parents back you up. Like has been said-money talks. I have also seen instances where BO/BM's take some liberties of sorts with the horses of teens that they would not think of doing with an adults horse. Just an observation- THe "kids" tend to let it slide whereas the adult horse owners are not so apt to do that.
Is this guy actually riding your horse? Or doing ground work with it? I am a little confused.