I wish you and your son the best.
Don't get mad at your son, but try to understand where he is coming from. If his true dream (at this time) is to open a pot shop, then talk to him about it. What will he need to do to make his dreams become a reality? It is not as easy as just putting up a sign and making it happen. If he plans to grow his own, he will likely want some horticulture training to really thrive and some business knowledge. If he just wants to sell, he will need to be able to come up with a business plan to get the loans to open a store, credit to run the day to day operations, know how to do some accounting, what it takes to get a license, and know the ins and outs of how to run a business. Will a bank take a kid that barely made it out of high school seriously? Not a chance. How about a college grad with a business degree/horticulture minor with some internship experience? Go tour the pot shops that are doing well with him and get him to talk to some owners/mentors what they did to get where they are today.
Use his dreams (even if you don't necessarily approve of them) to motivate him to be his best self. Worst case scenario, he becomes a successful pot business owner. Best case scenario, he discovers other interests along the way to pursue.
Also, try to spend more time with him doing stuff (i.e. trips, hiking, biking, fishing, projects, etc) and just enjoy his company. No lecturing. Invite his friends. They aren't going to smoke up with you around.