First of all, you should know that any money your kid ever earns is legally yours until he earns it at 18 or older. So enough about this nonsense about him being paid back. That's crap. he's a minor and you pay all his bills.
Secondly, don't ever let the argument fly that drugs are fine, because society has smoked enough to destroy its collective brain and decide it should be legalized. It's still against federal law, and it's still against the law for anyone under 21. So, it's against the law. he could be arrested- and so could you, for allowing it. he could be in foster care. he could be doing community service. And he could be in juvenile detention or jail if he supplied any other kids.
As far as him doing drugs, this can and will if you let it destroy his progress in school. If he's having trouble in school, get him a tutor. The only thing smoking marijuana will do for him is ruin his brain and cause him to be even dumber than he is now. He'll loose ambition, flunk out and then you'll have a bigger problem.
You need to be a parent. Flush it, take away his resources. YOU control his rides anywhere. YOU control his access to his phone, the internet,games, his music, his movement anywhere. SO do that. Make it clear this is not acceptable, and restrict him for a while until he understands how much in charge you are. Then hold him to it.
Or you can let your son fail and do drugs.
Your choice. His life.