When he walks, his left leg is always out to the side when they cast to go behind him. I would suggest an FI exploration where he has to lie on his left side with his left leg bent and roll himself forward and backward over the left leg placing his right knee in front of the left knee and behind the left knee. This will create a constraint in which the outer edge of his left leg is prevented from bending out away from under the center of him. He'll have to acquire the capacity to bring his left leg behind him so that it's still under him, under his centerline.

Just a thought… another idea is to take him through the "dead bird" ATM, with his left leg bent behind him (right leg bent in front). With his left hand reaching to the right, make sure he keeps the inner edge of his left heel in contact with the ground as he rotates the right. This constraint will get him to engage the extension of the left leg, and the heel detail will help him find support along the medial edge of the left leg. You can even sit at his heel and give him a hand to "push off" against; or better yet, arrange him so his left foot is in contact with a wall.