@Jerseytwerp,
Yeah what mame said. And it isn't a failure on your part. It can be simply him being a teen and trying to fit in, experimenting, etc. It is actually the opposite as you notice this change ...that is,an indication of good parenting.
I think talking to him with your ex would be best. Focus on by being supportive and being clear on how this impacts the developing mind at his age. One thing is with the legalizing of pot...might make,many kids think it is not so harmful. But it is,especially to teens .
Another twist that could be on your side...is your son involved in sports? If so this is leverage...he could be kicked off the team...it impacts his athletic ability
One thing mame asked is what us his age this could change how you deal with it...if he is 13 you would handle it a bit differently than if he us 17.
I would not be against grounding and removing privilege s depending on age and circumstances... But I think first you need to really see what is going on and move more to his buy in.If he has been communicative with you before then you should hopefully have more success showing you care and want to help .