Sit back and drive him up from behind, as everyone else has said. Give him a bump with your legs every stride or so, and drop those reins. If he's a WP trained horse, you shouldn't need your reins anyway. Put them loose and in one hand, until there nothing for him to lean on.
If that doesn't work, get those reins and bump until his nose comes in and tucks. Then give him a release. WP/reining/cow horses are meant to be ridden on a loose rein. You don't need to be giving him any contact unless he's a greenie. He needs to find the soft spot and learn there is discomfort in having his head rooting out.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.