Churchill once said, when asked how to give advice to his grown kids, "Find out what they intend to do and then advise them to do it."
There are a couple of things that are essential to helping a kid on drugs that our society has got dead wrong.
First, 15 - 21 year old people are not really children. They're very young adults. Call them teenagers or kids or whatever, but they are not mentally or emotionally children. Though they don't have everything figured out just yet, it's not only unwise but usually ineffective to try and force them to do something they don't want to do or to prevent them from doing something they want to do.
Second, not all drug use is a problem. Even when it is a problem, the user has to be the one who sees it as such and wants to improve things; either to quit, switch to something more effective or get control of their use. You can't force anybody to do this, it's got to be their own idea. If the kid isn't inclined to seek help then the best thing you can probably do is to leave it alone so that it doesn't become a point of contention.
Above poster is right, in my opinion. Education is the best way to deal with most problems. But it's got to be accurate information. Avoid any publications by Miller Newton, endorsed by Partnership for a Drug Free America or Oprah and, especially, anything endorsed by AA, NA or other stepcraft organizations. Try Vaults of Erowid or your friendly neighborhood pharmacist for accurate information.