D.B.
He's doing it for a few reasons: he's addicted (at least emotionally), he's able to afford it, and he doesn't have meaningful consequences.
You've grounded him multiple times - but it's not working. So you have to identify what he values. I'm guessing he values his computer, his phone, and his allowance.
He wants to be an adult? Great. He pays for those things. And if you aren't locking up valuables, start. Buy a fireproof safe at any office supply store, and put your jewelry and your wallet in there unless you are out of the house. Also any prescription drugs you have for yourself. He may be selling those. He loses his phone - because apparently he's using it to arrange for someone to purchase weed and juuls. He loses his computer unless he's at the kitchen table doing his homework where you can see things. Otherwise, he can go to the public library and use it under the librarian's supervision. I'd stop imposing things you cannot enforce. Yes, you can ground him but if you aren't watching him every second, he'll get resentful and be determined to beat you at this game. You have to work with him in areas where you have total control, and that's the finances.
Privacy - he is a minor, and he has no expectation of or right to privacy, short of you not opening the bathroom door when he's in there. That's it. His texts, his emails, and the contents of his room (including his bank account and the cash in his wallet) are your business as long as he is living in your house, a house you pay for. When he goes to take a shower, go in and go through his things, as you have been. I think random drug tests are one way to go. You cannot announce them, and they have to be done at the moment he wants something from you - like a ride someplace or an allowance or something else. And absolutely hold over his head the idea of a learner's permit and a driver's license - no way he's getting behind the wheel of a 3000 pound death machine if he's impaired or using substances it is illegal for him to purchase and use. If he breaks one law, he'll break others.
I think Lori's suggestion about having him talk to a police officer is a good one. We did that with our son when he did a dumb but inappropriate thing (following the lead of other kids) in 6th grade (not drug related). The principal called us all in, and said he wanted our kid to also talk to the school resource officer. The SRO wisely suggested we meet with him at the police station, not the school. He had a talk with our son about integrity and respect and so on, and basically said he didn't want to have a reason to see him there again. Your son is older and may be more jaded and defiant, but being on the police department's "radar" could be very sobering. I also think counseling would be important - and yes, you should all participate. If you are on the same page as your son's father, even better. I think a unified parental approach on a plan you agree to with an objective counselor will be better designed and easier to stick to.
Taking into account his college or other after-grad plans is one part of the approach, but kids under the age of 25 don't have the brain development to fully predict consequences yet, so they often just don't see the repercussions, or they don't care.
I agree with you about medicinal use, and recreational pot is now legal in my state. But there are plenty of things that are legal for adults that are not legal for kids and teens. So draw the distinction there. So I wouldn't have the "it's harmless" argument - I'd have the "you're a minor" argument.
You're wise to get on top of this. Don't let it go. I don't entirely agree with the "gateway drug" thing in terms of physical addiction, but I do worry about the reasons why some kids are smoking or vaping all the time and what their psychological draw to it is. If they have no way to afford it, the next step is stealing and shoplifting.