Family members outside of parents/siblings are sometimes seen as being somewhat more neutral. Take him out to lunch, visit with him about what you know, and what you're concerned about. I've had these conversations with family members (and a few missionaries in my mission) and I think it really works.

Growing up I had a number of friends who dealt drugs. For some kids it's just a phase and they'll eventually move on. In many cases parents already have a hunch that things aren't right. The issue is that your family member isn't likely to change until they have a desire to. Conversations that result from a third-party tip rarely catalyze the change of heart required to change behavior, and can sometimes do more harm than good.