Well my best mates sister who was 40, and a good friend died of lung cancer and it spread like wildfire.. from a diagnosis at xmas she died 3 month later this year. She smoked weed everyday, and had a pain in her side all the time. But smoked weed more to get rid of the pain. But that obv was the cancer..
You can give him as much advice as you want, my mums always truking people who have died in my face from coke, which i dont blame her, but its not just going to make me stop like that. If you new some1 died from drinking today, would you not drink again?
The main thing he really needs to want to stop himself. He might have to hit rock bottom and have enough of it. But just make him aware, you dont get motivated enough and do anything with your life, and you get real bad paranoia.