When he goes to do it, give him something else to do. Speed up, turn in a circle, ect. Give him a strong "This is what I'm TELLING you to do, not asking" cue for each of these things.
If that doesn't work, carry a crop and tap him on the shoulder or neck when he goes to do it. After 3 times or so that he tries it and gets a light tap he should get the message. Posted via Mobile Device