Thank you for the replies to the questions and the added information. It helps a lot in understanding what the situation is. I believe I can now be of help with this issue.
First, let me say I can imagine how difficult this situation must be for you. On the one hand your son has been on track to be successful in his school career and adolescence in many ways. But now this might all be derailed because of marijuana.
And this is actually the key to my answer to you that you need to consider and think about. I urge you to reorient your concern away from the dangers of marijuana's safety and gateway possibility to the more pernicious problem: marijuana is a turn away from the direction his life was taking.
Here's my point: whatever arguments you are going to present showing the cognitive impairment of marijuana, the psychological addictive qualities, the linkage to subsequent use of more addictive drugs, etc. he will hear the opposite arguments from friends, online sources, or schoolmates and come back to you with how biased these studies are. And he'll find tons of websites to give him arguments to come back at you with: how alcohol is so much more addictive, how it's so much more dangerous in and of itself without having to be a gateway drug, etc. etc. yet you and dad drink. And where will you be then? In a weaker position than you are now.
So, please, abandon that as you argument. Use it only as personal opinion to share with him. But if you need a book for yourself, the best is Marijuana Myths and Facts: A Review of the Scientific Evidence by Zimmer and Morgan. It's readily available in libraries and online. It's too old for the science now, but the science really hasn't come up with anything dramatic since 13 years ago that proves anything one way or another. It's the most balanced view on the drug.
But again, this isn't the problem. You must recognize what your concern really is and communicate it to him: your concern is that he is derailing his ability to succeed in adolescence. How?
Adolescence is the time where a person chooses what his VALUES are going to be. Values such as ambition, character, religion, patriotism, sex, love, honor, goals. All these are values. Yes, including goals. How so?
Are his goals going to be "the moment's pleasure"? That's marijuana and the people who indulge in it. Or are his goals going to be hard work, putting in effort. Those are people who are studying, on sports teams, at church, etc.
Same with all the other things. Character. What is it? Does hanging with the pot heads build his character? Are they people of character? Where ARE people of character to be found?
Do you see how THESE are the important questions he has to ask? Well, you have to engage him in those questions. Because that is your weapon against the allure of the pot parties and online chats. So, have these discussions with him. Explore your own thoughts on these and other values with him. Use this as your teaching moment for your son's soul.
I don't know if your daughter is a good person to have in this discussion or a future discussion. She has gone through it. She knows it from the inside. I don't know if she is through it enough to speak with him about, nor do I know their relationship. But on the face of it, she has an important perspective in the family on this problem. And sharing it openly in the family with her younger brother may be an important step for her as well as for him.
Okay, I wish you the very best!
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