When teenagers start smoking at a early age they usually lose motivation to finish school and give in to peer pressure of all sorts. I started smoking when I was 15 due to heavy peer influence and since I didn’t have a dad at home I gave it a try and it became medicine for me to Escape reality. It’s very hard to stop smoking once you start so I will be praying for you momma! Praying to get your son involved in more activities take him out to dinner just be there to let him express his thoughts. Let him know you care for him and no matter what he chooses to do you love him and want the best for him. Pray pray pray if your not a believer then let this be an opportunity to get to know Him.
Put your foot down! You’re mom! If there should be a next time leave a bad taste in his mouth about how you feel about it. Always remind him you love him and as a mother just want the best for him, and on another note, show great admiration and value to the honesty he has with you. Let him feel your energy to both points. And be sure that’s what he was high from, do a over the counter drug test if possible, lots of drugs bring on those effects.
My family is full of people that smoke. I have before. Their father does. Their step father does etc. My 13 year old son, ever showed up high…I would have his ASS. We would be having a long talk. He would no longer be having those friends. Or privileges of any kind. Yes, weed is not as bad, or even close to any other drugs or alcohol. But, he’s barely even a teenager.
Find out why he is smoking first without punishing him. Kids will experiment. If he tells you it’s because he stressed, struggling, depressed, etc then take him to counseling and if you’re in a legal state see about getting him a card. If you’re not in a legal state, explain to him that you both can get into some serious trouble if he gets caught and so could his friends. If you get mad, he’ll do it more because kids rebel. If you educate him then maybe you can convince him to wait until he’s older to smoke it.
The best way to break addiction of any kind is to be around people. Keep him busy and working
Some may not agree with this and honestly I wasn’t as young but when my parents found out and I wouldnt stop they made it a rule that I was only allowed to do it at home occasionally and not drive…also it couldn’t affect my grades. It worked and I wasn’t out driving and crazy and under influence and peer pressure of other people. We were open and I didn’t have to lie or sneak with them and we have great relationship
Thank the Lord Jesus he ain’t smoking crack! 
Get used to it it’s legal now. He’s not going to stop
Healing with Cannabis: The Evolution of the Endocannabinoid System and How Cannabinoids Help Relieve PTSD, Pain, MS, Anxiety, and More https://www.amazon.com/dp/1510751866/ref=cm_sw_r_cp_api_glt_fabc_XW200WJJSH3DDRDDF9J8