EDIT: This turned into a bit of a long post so I've done a TLDR.
A large number of the replies on here are completely unrealistic IMHO. All these 'he's a child, ban his pocket money' types of post.
He is a child technically but kids grow up quicker than ever nowadays and ultimately he's less than a year from being legally considered an adult. He can smoke, work, and up until recently would've been able to get married.
The truth is that you can't fully control a 17yo if they don't want to comply. In many cases they're conflicted/challenging authority but still not independent enough to want to fully rebel and burn their bridges so they may relent a bit, but ultimately you can't really control them if they dig their heels in - look at all the threads of even younger teens running away to friend's houses and the police not intervening.
I started dabbling in weed at 16 and gradually increased my usage until I was smoking it daily at 18. Was from an educated professional family and had a good upbringing but always rebelled for some reason, likely because of my overly controlling mother. My sister was the golden child, always did as ordered and ended up going to Oxford uni.
My mum tried taking away my weed on multiple occasions but I just told her to fuck off each time and went and stayed with one of my older friends for a couple of weeks. Ended up locked in conflict with her (my mum) and bombed my A Levels (predicted three A's but dropped a subject and got CD).
Got into a pretty average uni and managed a 2:1 mainly through last minute cramming, and then went into a graduate job. However, after nearly a decade I moved into construction which much prefer.
My main point in mentioning the above is that I encounter a lot of young lads in the trade sector that smoke weed. It's obv not ideal but I think it's much less of a handicap than in a corporate role. In the latter, you often need to be an effective networker and good at communication (presentations, speaking in meetings etc). You need to come across as 'the right type of person'.
I found a lot of my mates that smoked weed into their 20s weren't really the most dynamic and didn't really advance as well as others in graduate jobs as they weren't as ambitious. However, my mates who went into trades didn't seem to be as affected as they were able to go about their day half baked - once you're adept at digging trenches with the digger or pumping cement etc it's not rocket science. And people don't care how 'dynamic' you come across as long as what you build is decent or your plumbing is good etc. They were on generally higher salaries in their 20s and didn't have tens if thousands in debt.
I noticed a lot of young lads smoke weed every day whilst working but then tend to give it up when they settle down with a girlfriend and get a house etc.
TLDR: I'm an ex weed smoker who did the uni/corporate job thing before moving into construction sector and working with tradies, and I found that lads that smoke weed often do far better in non office/corporate jobs.