and on some days I think it if all be better to just end it all so I don't have to see him and up like his dad
Please don't! I can think of countless reasons for you to get that thought out of your mind, but I've got three that are sticking out:
Your son loves you. That's what sons do, they love their parents even through hate, anger, spite and angst. Losing his mother'll likely devastate the boy, and you've worked so damn hard already to keep that from happening that I think it'd be a shame for you to give in and add to the source of whatever's bothering him.
Most people, in my experience, end up experimenting with drugs at one point or another in their life. For many, it's just a phase-- they'll go hard at it initially, then gradually the drug'll lose it's "magic" and they'll eventually end up putting it down for long periods of time. Again, that seems like something you've been waiting for, angling for, trying to nudge him into the direction of.... and I think the day that he makes that decision will be a wonderful one for you, a day you won't want to have missed.
Lastly, Klonoa, I don't know you but it would break my heart to know that you've posted this and followed through. I don't know much about raising children/teenagers through these kinds of situations, but if you ever need to talk I'll do my level best to be here for you-- even if you just want to vent and need somebody to say "Oh my god, that's horrible!" every now and then.
So onto that actual question you posed, it's a tough one that's gonna require a tailored approach depending on your boy's personality. There's no one-size-fits-all solution when it comes to drug use (or abuse), but here's my take on it and what would probably have worked on teenage-Me:
You could just be totally up front with him from here on out about drug use (more specifically,
your personal experiences with them if you have any, good
and bad.) Explain to him your specific concerns, and then give him real-life examples of those fears coming to fruition for other people so that he understands you're legitimately worried about his health. Talk to him about
his experiences, but be careful to stay as neutral as you can if he opens up to you. Judgement is okay, we expect that from our parents.... but we have to understand and relate to
why our parents are upset with us in order to take it as anything other than an all-out attack on our preferences and, by eventual flawed logical extension, our personality.
You get him to open up about something that obviously interests him, and hopefully you'll find out why he enjoys it (and that could be as simple as "It feels good, Mom," but there may also be reasons that you're totally unaware of.) Through discussion you test his perspective and, hopefully, over time you'll be able to re-educate him in a sense. You won't be able to get him to do a complete 180 but if you can get him to really empathize with what's going through your mind I'd be surprised if something didn't change for the better. I mean, nobody wants to lose their mother, and I'm pretty sure that goes double for a kid that still lives
with his mother.
Keep your head up, Klonoa, and stay strong. In most countries, marijuana use won't ruin your life and it really seems like a coming-of-age thing, in my opinion. I've always thought that if and when kids get interested is probably the perfect time to truly educate them about the ins and outs, the pros and cons of drug use, but I guess that kind of thing would vary from one culture to the next.