Tell him that everything in life is good as long as you dont abuse it, alcohol for example can become a very dangerous drug if you abuse it, that doesn't stop anyone from drinking occasionally, explain to him that if he smokes too much he wont appreciate it as much and might lose drive and interest in life. If you scream at each other its not going to work, you need to calmly explain things.
Cannabis is not that bad, I used to smoke everyday when between 16 to 23 years old and i don't think it really had a negative impact on my life, at 23 i stopped eventually cuz i got bored of it really and realized that cannabis don't bring anything positive anymore to me. Also explain to him that he should avoid hard drugs, maybe show him some pictures of addicted people that went really bad. Communication is key, if you just shout at him and tell him that you are against it it is never going to work.