Have the lunge attached and lead him around have the lunge whip in your inside hand, the line in your outside hand. Keep the handle of the whip to his shoulder. Slowly move to the inside by a step keeping the horse away with the whip handle. Just a couple of feet to start. And then slowly increase the distance. When he is happy walking six or so feet away from you, still walking get him to go around you in a circle with you walking a large inner circle parallel to him. You can drop the whip behind him but keep him on a short line so you have control. Once he has the idea you can let him longer.
Lungeing a young horse is very strenuous on their joints so keep the sessions very short 10 minutes is ample, and keep the circles large.