I'm really worried about one of my friends, what should I do?

One of my friends has recently met some people and started hanging out with them. From what he tells us, they don’t seem like good people for him, or anyone, to be spending large amounts of time with. They’re pretty much the only people he hangs out with besides me and my friends, and he’s with them very often. He tells us they might possibly be doing drugs, and I’m worried about how they’ll influence him. I’m wondering what I should do about this. I’ve already told him that I’m worried about him but he won’t seem to listen to me. I need your help in dealing with this.