The only thing I've found is repetition. As soon as he goes to rest his foot, get after him. The horse should understand that when he has his chain on, it's time to work.
In the picture I would have asked him to square his front end up anyway which should have resulted in him balancing his back feet.
You don't have to make it a big fight, but just practice. As soon as he rests you should ask him work (say a pivot or back) and then square immediately. Then go back to your position.
Sorry I don't have any magical advice aside from practice