He's an adult. He can make his own choices. You don't control his person anymore.
You do have control over your home, so if he is living there you could not allow him to smoke pot on the property, but realize he'll just smoke pot someplace else with increased risk of arrest.
Is he finishing his high school diploma? Is he working? Lots of people--productive, healthy, bright people--smoke marijuana regularly. We're not talking about a physically addictive substance like H here. Pot doesn't make nice people into junkies. Sure, you'd like your son to not smoke pot, but once your child hits eighteen he's like the Honeybadger--he can do what he wants.
Also, take some consolation in the fact that statistics indicate most men who smoke pot between 18 and 23 taper off in use in their late 20s. And probably the worst thing that might happen to an eight-year-old from hanging out with your son when he's baked is that he might develop a particular affection for Goldfish crackers and the music of Pink Floyd..