When the dog starts pulling that means that his attention has been diverted. He is focusing on a smell or something which has caught his curiosity, perhaps just being playful and trying to run off.
When you stop, the dog is "brought back" to reality, and when you move again, the dog's attention is back on you for that time being until his attention wanders again. I have to say that if just stopping and going is enough to correct his behaviour, you are on your way to a well disciplined dog.
Just a suggestion, to increase his attention on you, keep working in short stints. I will leave my dog in heel while walking on a leash so that his attention is on me all the time. In order to do that, keep working the dog in heel-sit-stay, heel-sit-down-stay, heel-down-stay, heel-down-sit-stay and all sorts of variations. This makes the dog understand that he has to keep his attention on you to anticipate a command while in heel.
Make sure that during your heel you incorporate wild, sudden turns to the left and right and do not hesitate to use your thigh to bump his head while turning left if he does not respond. This will teach him to learn your body language.
Hope that helps.
Regards,
TH