I'm all for adults smoking weed recreationally, but teens smoke weed and get lazy and their grades drop. I'm sure it's also not all teens, I smoked and still did great in school, but if this parent is testing their kid there's probably a problem. The best thing to do OP is to teach your kid to use it responsibly. Or to act like it's totally cool, and see if he just gives up on it. There's really no great parenting answer to getting your kid to stop doing drugs.

Parents are testing their kids for drugs in the US? Sounds weird o.O Also, doesn't seem like a good way to deal with the problem itself. If your child takes drugs or you think so, you should work on the relationship so they know they can turn to you for help, not just controlling them or handing out punishment if they "fail". It's hard to work with teenagers against their will if they don't want to quit. My English is too bad to really describe what I mean :-(