I would just sit down with the 17 year old and have an adult conversation with him. Explain both the upside and downside of drug use. Just the facts not rhetoric and allow the kid to make an educated decision on what he thinks. And be sure to ask for his thoughts on it. As an adult we have a different viewpoint on a lot of issues but its important to see how the kids are coming to there conclusions and maybe try to help them see where they might be making errors in judgement.