Hi Wren, and welcome to this forum.
My son also started with alcohol and then pot. I know there are a lot of points of view about pot, but my view is that it's just not a good idea.
Especially for people like my son, who have addiction in their families and evidently (by his subsequent behavior) proved to have the addictive gene in full bloom.
I think one thing about pot smoking is this: It's not just the pot. It's the behavior that often seems to come right alongside it, the lack of motivation, the lack of congeniality with others, etc.
It's as much, for me, about the behavior as it is about the illegal substance my son was smoking.
My son lived here with me when I found out about the pot. I told him, no way, not happening here. If you do it, you are out.
Ultimately, that's not what ushered him out the door---his out of control behavior with alcohol plus his blatant tearing up of yet another contract that I wrote and asked him to sign right in front of me---that is what led to my saying: You can't live here anymore.
He walked off down the street, and I thought I would die.
But I didn't and he didn't. My son's story isn't your son's story. But one thing I learned (finally) is to keep it simple, set clear guidelines for behavior in my home and stick to it. Don't make it real complicated for you or for him. You can do that later.
I am sorry for your confusion and your pain. It's really hard when this kind of stuff starts happening. My son went downhill fast and it turned out he was dealing with things much more serious than pot.
I hope that doesn't happen for you.
You can't control him, as I am sure you are learning. What you can control is what you allow in your home.
My son was very hard to be around for a long, long time. Finally, today, things are much better, and he is rebuilding his life. It's been a long six years.
We're here for you, to support you in whatever you decide to do, and to offer options and ideas. Keep posting. We get it, and we care.