Have some patience and persistence. If you decide to take on such a difficult case, you will have to endure much suffering, because the influence of an adult are not so simple. Tears, cries, and pleas in such a cause is unlikely to help. The ultimatum "it's me or Smoking" does not always work, and, uttering it, you have to be a hundred percent sure of the guy's feelings, otherwise you risk to be disappointed.
Talk to the guy on the souls. A simple conversation can bear fruit. Try not to make claims, accusations, don't scare him green children and other myths. Express your concerns, share fears and doubts. Your anxiety means a lot for a really loving person, and in this case, he definitely will listen to you. Say what you will to support him and help in any ways to overcome the addiction. So you will set your empathy and show that he is not alone in such a difficult situation.
Ask for help to his friends and relatives. Sometimes to influence the addicted person need not directly but indirectly. Find out who your young man is an authority, and turn to him. It is important that this person was not a supporter of authoritarianism and managed to act on your boyfriend's conviction, not by force.
Persuade your young person to seek professional help. If his addiction had gone too far and poses a real threat to his health and life, have to resort to expert advice. Persuade the guy to go to the usual appointment with the psychiatrist or at least psychologist. Both doctors are able to help him realize the problem and begin treatment.
If you intend to build a long relationship, to marry and have kids with a guy who smokes marijuana, you should think about the appropriateness of their choice. In case of unsuccessful attempts to wean Smoking illegal substances, you risk to connect his life with drug addicted person, children which may be born with serious developmental disabilities.