If this is for flying lead changes, I like to do a lot of turns on the forehand, and expanded forehand turns.
Expanded turns though are my favorite. I like to do them at the trot but starting at the walk is good. Just ask him to walk up, then push his hip to the outside so it makes a 90 degree turn. Don't lose the forward motion though. It gets that leg reaching out and also will help unstick the button when you get up to the lope. By the end, you should be able to trot a square. Go about eight steps, push the hip to the outside 90 degrees, trot another eight steps, push that hip, etc. :)
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.