I agree with Fluffy - talk to him! Catch him first thing in the morning, when your moods are good...
I've been smoking weed for 16 years, almost daily, and I've had a great career, good friends, a house and half acre, my car and a little truck, and a wonderful husband and a kitty. But that isn't to say I don't have serious, serious problems and self-loathing..in fact, I'm not even sure how I got to today. I haven't dealt with these problems until just a week ago, and then I broke down sobbing when I realized why I'm this way...childhood abuse. It was all in a book I read.
There's probably a huge, huge reason why he smokes pot like that, which he isn't ready to admit to himself. It may take forever, and he may never quit. If you go behind his back and or tell on him to his parents, he may not trust you anymore. Have that talk with him...to his face. Respect!

I can say that after I finally admitted to myself that I was abused as a child, I've been smoking a lot less pot in this last week than I have for the past 16 years. I'm just not that interested in it anymore...to kill the pain and numb myself. I'm curious to discover who I am, and what I could be if I stop wasting time doing that.
That said, Dying Ember is right - people have been using marijuana for thousands of years as a true medicine, and just because it's illegal doesn't mean that it's evil. Alcohol was illegal in this country less than a hundred years ago. That's just what the law happens to be today.
Wondering how young he is?