Make his mouth very soft and make his back round. If his head is up in the air, he cant lift up and change his hind leg. Make sure you are going slow and ask when you turn. Try even using your outside rein and step out.
I use a dressage whip to help him
Did you try to help him understand by demanding the change through little jump or putting a rail down? As he would try to jump it at first it would be easier to get the change