As Bobby has settled, he has started to enjoy lunging and loose schooling more

This has turned in to a double edged sword - I find it very hard to keep him in walk - he loves to trot, and this is his pace of choice, both loose schooling and on the lunge.
I don't lunge with a stick, so use posture and voice aids.
I am finding that I am really struggling to shift him down a gear from trot to walk, and to keep him in walk - particularly on the right rein. Canter back to trot is easy, but he does love to trot!
Any ideas?
