I suggest focusing on how life can improve through quitting and that life is in fact much, much better when you stop.
Do some research and you will find plenty facts to use that support the above. Most if not all smokers at some point believe that they will always be miserable if they quit.

There are now MORE ex smokers then smokers in the UK, USA and Australia.
Tens of millions have successfully quit and won’t ever go back.

http://www.achoice2live.com/ex-smokers-now-out-number-smokers-in-us-uk-and-australia/

Hope that helps
References :
http://www.achoice2live.com/ex-smokers-now-out-number-smokers-in-us-uk-and-australia/

Does he waste all his money on it? Does he smoke so much that it is holding him back? Is he using it to escape from his problems? Because those are all legitimate reasons to quit weed (at least for a long time, until the problem is solved).

However, do you not like it because the government, PSA’s and fake studies tell you it’s bad? If he is living a normal, happy life, then I think you could stand to be a little more open minded. I hate to beat a dead horse, but marijuana is far less harmful than smoking cigarettes and drinking alcohol, even though the idiotic US government places it in the same category as heroin.

So, if there is a real problem, like the ones I mentioned above, simply explain to him how it is being detrimental to him, in a calm way. Make sure he knows through your words and your tone that it’s just because you care about him, and you noticed a problem he was having. Never insult him or say anything that could be percieved as an "attack", because that is one quick way to get him to not listen to you. Instead, frame it as a positive thing. It should be more of a tone like "oh, look over here, this looks pretty awesome" rather than "Don’t stay there, that place sucks" if you get what I mean. Try to do way more of the former and less of the latter.

However, it just sounds like you are opposed to weed because of all the negativity surrounding it. If you want a little truth, you could watch a documentary called "The Union: The Business behind getting high". Or you could also do some research of your own. And no, not on any anti-drug sites either, because those statements are completely biased. As a marijuana user, I know from first hand experience that they completely exaggerate the harmful effects of marijuana to the point of flat out lying and fear mongering. You CAN use marijuana your whole life (after you’re 18/21 when your brain is developed enough) with no harmful effects at all, if you do it right (vaporize instead of smoking, know the right time to use it, as in, don’t show up to work stoned, make sure you budget right, if it’s illegal, keep it hidden, locked away, maybe in a scent proof bag, etc. etc. just common sense, personal responsibility stuff)

Remember, weed itself isn’t harmful. It’s how you use it, that can POSSIBLY be harmful. So if he has a problem, address that it’s HOW he smokes weed, and how he is using it in a way that is not good. However, the same way society views occasional drunkenness as an O.K. thing (though I think drinking is a pretty nasty thing to do to your body), marijuana can be much safer, more enjoyable, healthier alternative if used correctly.
References :