Unfortunately I have no one on the ground to help, and whatever I do I can't persuade him to just move one leg!! I've tried using my legs by nudging or squeezing, shifting my weight, tapping with a schooling whip but nothing seems to work. If I get too insistant he goes backwards or sideways which is not something I want to encourage when I am trying to get a good halt.
I think I'll have to teach him in hand first and hope I can transfer it to under saddle.