If you have help available, have someone hold his halter and lead him around the circle to the right while you are standing in the standard lunge position.
If you are by yourself you can do the same thing. Holding the lunge line so it can pay out, lead the horse to the right, of get the horse to walk next to you without holding the halter, and slowly let out the lunge line while maintaining the walk and staying in the correct position you would be in to lunge...behind the shoulder. It may not happen immediately..the horse will probably stop, maybe turn.
If you have a small enough paddock you can try free lunging, similar to round penning as the lunge line is not attached, concentrating on keeping the horse going to the right as much as you can...when the horse does lunge to the right give a LOT of praise.