After tacking up I worked him both directions and got him to canter pretty nicely each way. Now to just get him to do it with me on him!
I finally decided to exit the round pen when a paint mare that was in a pen right next to it kept coming over to Willy with her teeth barred and ears back. Each time around I said NO sternly to her and flicked my whip are her. Still it's not really a conducive environment to getting work done when another horse is trying to bite mine!