If he is pulling on the lead then his attention is not focused on you. You must develop eye contact with the dog so that he looks to you for instruction rather than trying to make him do things.
I don?t often advocate the use of treats, but to develop eye contact they can be invaluable.
Teach him to sit using your voice and a hand signal. Lure his head with a treat concealed in your hand, and when he is looking at the hand, which should be positioned in line with his eyes and yours, then give him the treat.
Practice this until the dog is focusing on your hand, then delay the treat and his eyes should be set on yours.
Once he has become focused, you do not need to treat him every time, but make him earn it.
Sit him at your side with a slack lead, move your right foot forward, tell him to stay and use the hand signal as before.
Slap your left leg as you move it forward and give the heel command, luring him to your side with a treat in the hand and giving it to him as he looks up at you.
That is the first step to heelwork. Gradually progress to teaching two steps, then three, then four, until he will follow by your side, focused on you and with no pulling.