I caught my son smoking weed for the 2nd time. What should I do?

I have caught my son smoking before and told him to stop, and he said that he would. Two months later I catch him again. I do not think smoking marijuana is in anyway a crime, but you can still get arrested for it, and that is why I told him to stop. It is in no way effecting his grades, in fact, his GPA has gone up since the first time I caught him. I honestly don't know what to do. I could let him smoke it in the house or at friends houses, where he would be the safest, where I can keep him from driving, and where it is more likely not to get arrested, or I could just do my best to prevent him from ingesting anymore weed. But in order to do that I would have to monitor him more closely, keep track of all the money I give him, and overall just oversee his life more, which I personally feel is a bit unethical, all to prevent something that doesn't affect anyone else. What should I do?