I read your post and my heart goes out to you. I learned my husband of only a few months was using *******. He realized it was interfering with work and home and ended up going into long term residential treatment. He passed last year from cancer related issues, but had been clean for over 15 years prior to his death.
Your loved one who smokes weed, do you know how long he's been smoking? Does smoking weed interfere with his personal and professional life? He is the one that has to ask for help. This generally comes after realizing how his actions are affecting others in his life either at work, home, friendships... You can give him suggestions on where he could get help, or the effects weed could have on him.
********* is legal in some states in the US, as you may be aware. Still, however, illegal in others. With that said, some states have even relaxed their policy on the amount you can have on you if detained by a police. Sometimes only a ticket given. Does this mean that it's okay to use weed, no, it means that states are focusing their efforts more on activities that are more severe and criminal in nature.
I don't know if I would be too upset if I found out my husband was smoking weed in this day and time, unless he was driving or operating mechanical equipment. If he were using it at home only, I would be a little more understanding. Only again, because states are legalizing it, law enforcement are relaxing their policies on the amount.
While I understand your concerns, have a calm conversation with your loved one and try to get as much information from him as possible. Ask him how often he smokes weed, when did he start, how did he get started, why did he chose to smoke weed, when does he smoke. If his responses center around dealing with stress - offer other ways to relieve stress like walking, music, meditation, helping others, working out. I think the more you two talk the better you can understand what's going on.
You may want to think about getting help for yourself to better understand your loved one and your role in this issue as far as being an enabler, how to deal with someone using illegal (I think it's illegal in your area), substances. Reach out to groups that work with family, friends and loved ones of addicts. Not saying he's an addict...
I hope this is helpful.