Hold your arm that you have the lunge line out facing the way you want your horse to go so you give him a place to go. The hit him on the butt with the stick slowly getting a little harder every time until he moves forward. Good luck!!
it may seem like an obvious thing, but make sure when you lunge him that you are driving him forward, meaning you say behind his shoulders more towards his butt to send a clear and direct message to move fowards. I.e, if you are working him in a round pen move with him in the center when his is along the outside and push him forward by using body language (hopefully that makes sense). Another thing you could do is go into an open area or arena and do close ground work and yeld his butt and make him go in tight circles for a while then ask him to lunge in a round pen or where ever you do your work. hope that makes sense and helps! :)