gsm,
no sense in beating yourself up about it.
Yes, you have set a bad example.
Yes, maybe he is trying to emulate you.
No, it is not entirely your fault.
I am a smoker but I am not defending you, merely pointing out that he is also subject to peer pressure from his friends etc but also from the media such as films, TV, magazines and of course subtle advertising methods.
The best you can do is what you have done so far and to teach him the dangers. You don't say how old your son is but regardless of that you can both get help together through the NHS and support each other.
How about striking a deal with him?
You pool what you save in a joint account, keeping track of who saves what, and if you both stick to your side of the bargain you get a reward.
Maybe he could get a new games machine, (playstation, xbox etc), you could get a new car?
But best of all if you both manage to do it the biggest reward you will both get is a healthier life.
Good luck mate and let us know how it goes.