and when he pulls turn around and walk the other way, and keep repeating until he stops...........what kind of collar are you using?
You could also try keeping him to heel with a treat - keep him on the left hand side of you, with the lead in your right hand, so you have your left hand free to give you some control on the lead and give it a quick jerk back until he gets the message. When he walks to heel tell him he is a good boy, some fuss and a treat - there is no quick fix and it takes a lot of patience, but if you persevere he will get the message.
You should also try obedience classes