I'm sorry that you're having a difficult time with your son.
It sounds as though your son is at a typical stage as teens approach adulthood ~ He is pushing his limits. Personally, I think that now is the time to learn the consequences of actions. So, yes you are paying your son to do his chores. Whatever he chooses to spend that money he makes upon is his decision. I'd recommend being up front with your son. Tell him that you suspect that he's using the money that he earns to buy marijuana. You can then tell him how you feel about that and why. Like: I really don't want you to be smoking. There are physical and emotional consequences of doing this drug. It's known to slow thoughts and reactions down, it is an illegal substance, etc...
Hopefully, your son will talk with you & not just avoid talking completely! Regardless, now is the time to tell your son where you stand on the issue. Like: "it's
not okay to smoke around here." Plain and simple. You can assure him that you do love him very much, but you stand by your decision to do as much as you can to keep drugs away from your son, in hopes of avoiding watching him make painful decisions and hurting himself in the process.
Obviously, it's a lot easier to tell you my opinion that to follow my words. My girls are still in elementary school, so I'm sure my time is coming.

I've been warning them for years though & they can see the effects right in front of us. Whether or not they choose to follow my wisdom I don't know.
You might want to try getting your son involved in feeding the homeless, at a shelter. That's an opportunity for your son to see what's on the other side of using drugs. He could see that they are still very human ~ wonderful and kind people. Some suffering bad luck, others suffering bad decisions, and the slim minority of "bad people". It might open his mind a little bit. (I did that in my last year of high school, and found it to be empowering!)
Best wishes sent your way ~ welcome to PC!
My girls are still young (and I'm dreading the teen years!)