You are most definitely not overreacting. The brain doesn't even stop growing until about 25.

I'd have an open, honest discussion with the boy about all of it, appropriate to his age, the addiction in the family, what it's done. And yes, hold that license over his head. Impaired driving doesn't mean just drunk driving...many people don't get that legal reality. One can be impaired by many things. I refused my alcoholic son permission to get a license because I wasn't going to be responsible for putting him on the road. He's 30 now, sober for almost 3 months and now working toward driving. He's also thankful I saved him from himself in that regard.

His actions are illegal, even in Canada where weed is legal, it's illegal for young people.

I'm a bitch, but what I did, when weed came around is drug test my boys. Sure did. Every week for a while. Fail, well, phone goes, that PS 4, see ya, next week we try again. All while being honest with them.