How do I stop my friend taking drugs?

Ok so one of my best friends has been smoking weed quite frequently lately (every day or 2 at the most) he has already been kicked out of school but finally managed to find an 'introduction to trades course'. I don't want him to mess this up like he messed up school and I think the drugs are the main cause. I have tried talking to him but he won't listen. How can I help him to help himself?