This answer is relatively easy: it is against the law to smoke marijuana and it is your responsibility as a parent to keep your child from breaking the law. Furthermore, it is scientifically proven that marijuana de-motivates people and delays their goal setting and constructive planning for the future. It is often a prime factor in a developing teen's "lack of ambition."
Let's talk about your technique: Your energy would be best spent in setting rules and expectations for him. Then you can reward him for meeting goals or give him consequences for not meeting goals. This way you are teaching him how to build towards a constructive goal. Getting "on" a child and nagging only confirms to him his failure and doesn't model a way out of his dilemma.

Recall the negative as well as the positive parts of your own drug history. When parents, particularly baby boomer parents, speak about their drug history, they often imbue it with a sense of nostalgia for "those fun and free times." It is just human to forget the painful moments and to remember a general feeling of youthful energy instead. But, if we choose to share our drug history with our children, we have a responsibility to reach beyond the general. Do some personal excavation work to piece together those drug-using years. Why did you or your husband finally stop using? Some users (or their relatives and friends) suffered genuine side effects of paranoia, or experienced hallucinations that were very unpleasant and had long lasting consequences. If you think back to your drug using days, it is likely that you will remember "being in a haze" for a period of several years. Or you might have remained in a lower level job while peers were moving beyond you. Or your relationships with friends and family might have been impaired by your use. The Alcoholics Anonymous groups teach that if you are using a substance, then your primary relationship is to that substance, rather than to yourself, your loved ones, your work or your dreams. What would you have done differently and what do you want for him?