Me, I was taught a couple of other things, in addition to outside leg back behind the girth. (And yes, if he ignores it, the aide should be reinforced with a crop.)
Also inside leg presses in at the girth and the rider needs to lean back, all while at a rhythmic sitting trot.
Leaning back matters, because it gets the rider's weight off the forehand (the front legs). The horse needs to lift his front to strike off at the canter. The inside leg at the girth causes the horse to bend a bit, encouraging him to pick up the correct lead (inside front leg).
Hope this helps.