You don't say specifically what you want to talk about, but even if it's just general education, I would agree with Wickett - you need an open communication channel and a basis of trust, or you will be wasting your breath.
Beyond that, I would say stick to the fact. Remember that he can look up stuff on the internet, so don't go down the exaggerated scare-mongering route (eg. You will die if you try it!) Drugs are quite scary enough without any exaggeration, so don't give him room to call what you say "parental hysteria".