You have to do something. As you know and as it has been said in the thread, smoking pot is not a healthy habit but it’s even worse for your adolescent guy.
I can understand that your husband has an idilic idea of weed because in the end, this is what he was taught by his mother but he should see how much he depends on it to cope with his daily life. I don’t think he wants this for your child.

Talk with him and tell him that you talked to a doctor about it and how harmful it may be for your kid’s brain development. Any habit a parent may encourage is gonna be of big predicament for their kids, and an unhealthy habit but that has a shortage recompense is gonna be even more appealing. He’s being harmful for your kid without knowing it.
He also has to think that he’s his father and not his friend.
Somehow you have to make him understand it so the cycle won’t be repeat.