It is not easy to explain the concept of mortality to a teenager. Remember that song from the TV series, "Fame"? There was a line in the song, "I'm gonna live forever." I think that's the way most teenagers look at life.
You say that his two grandfathers and one grandmother died from heavy smoking and "he knows it." He may know it, but did he see them in the last stages of their lives?
I have a suggestion that is somewhat drastic but it may work. Find out through a friend who is a nurse or doctor if it is possible for you to get him a volunteer job a few hours a week in a terminal cancer ward, preferably lung cancer. You can also include an ICU or CCU, i.e., a ward with serious heart patients. Offer him some kind of reward in return, something that will be meaningful to him. One month in a ward like that might do the trick.
Alternatively, find some literature that shows graphically or that describes in text what happens to a person's lungs when he smokes.
Look also for other parents who are willing to be part of a support group to help their kids stop smoking. You can also speak to his guidance counselor at school. They may have already dealt with this problem. And who knows -- there may be other kids out there who want to stop and don't know how. Why not a support group for kids?
Lastly, but perhaps what you should do first, send your son to Wyatt in our Teen Center. He has an article there called "Wyatt on Smoking." Wyatt speaks your son's language.
Let me know if any of this works.