You need to teach your dog the correct way to walk on a leash

And to do that, you need to be the leader, not the dog.
What do you do currently when he starts to pull?
The most effective way to stop a dog pulling is to get it through to him that he does not (ever) get what he wants by pulling. Generally, what he wants is to go somewhere that you weren't heading or to get there faster than you were going. So as soon as he starts to put weight into his collar to pull - you *stop* walking. Make him come back to you (preferably behind you), and don't start walking again until he is calm. You may only get a few steps, but as soon as he starts pulling again, just stop. Yes, it's tedious and takes an age to get anywhere, but it *does work*. He needs to learn that the walk takes place at your pace, and in the directions you choose. If he doesn't comply, the walk stops (ie. there are undesirable consequences for that behaviour, and *never* a reward).
As he starts to get the idea, you can introduce a bit of a warning "ah-ah" and/or a gentle 'tug' on his lead when he starts to lean into his collar. Note that that is a *gentle* tug, not a yank or a jerk - we're just aiming to recapture his attention here, not punish or hurt the dog (which wouldn't help at all). He'll probably ignore it at first - so you come to a stop as above. After a while, that 'warning' should be enough to remind him of what you expect and the consequence of pulling - no walk. If it is, give him a quick word of praise or a pat, and keep on with the walk. Anytime it isn't, you just stop dead.
If the dog starts pulling as soon as you leave the house - well, you can just turn right round and go home. That's the worst consequence of all for the dog - the walk just doesn't happen
Do remember never to get into a pulling war with your dog. You will lose.
Give the above method a try - be absolutely consistent, and never let him be rewarded for pulling (either by getting somewhere, or by you pulling back). When there's no reward, only adverse consequences, the pulling behaviour will eventually extinguish itself. Doesn't mean he'll never try it again, but as long as you *always* come to a dead halt when he does that, he will quickly comply and walk nicely.
There are plenty of previous threads on pulling, and a lot of good advice previously given. All available just by doing a forum search
