How to teach a horse to lunge?

Are you wanting to lunge him on a line, or in a round pen? If he truly doesn't understand how to lunge, then in a round pen would be ideal. Either way, you always stay in line with their haunches like you are driving them away. Use a whip to encourage him to move away from you. If you are using a lunge line, start the circle out small and slow and as he gets the idea slowly let the line out longer and keep encouraging him with the whip until he's going around at the end of line.