And in addition to the above advice, when you put the inside leg on, you can move the outside leg off , just while he figures out what to do. Ulitmately, it would "drape" onhis side, a bit further back to hold the hind end in, but to help the idea of "move off the inside leg" you can open the door on the other side , the outside, which kind of invites him into that space.
Also, do not put the inside leg on nonstop. YOu kind of "pulse it on" with each step the pony takes with his inside hind. You will need to learn how to feel when the inside hind is stepping forward because your goal would be to have ti actually step forward and UNDER the pony's body, which give the lateral movement.