I really don't have the experience and knowledge to talk about family matters.
But there is no exception for pot. It may lack a physiological addictiveness. It might be "less bad" than alcohol and harder drugs. There is still a real component of addiction though, even if it's more akin to a food or sex addiction than a chemical one. And long-term and heavy pot use does affect the brain, moods, etc. Not to mention, he's spending money you guys don't have ("continued use despite negative consequences.")
But yeah, I would also think that there may be something underlying the pot use. I would check on resources for getting the two of you into some sort of counseling. Does sound like he'd be too willing, but I think it's rarely that people ever go to counseling when there's great communication.
Good luck.