Caught my teenager smoking weed?

My son is currently 15 years old. I've noticed that he's been withdrawn from everything for a while now. He's a straight A student, but I never see him hang out with any of his friends and he barely comes out of his room. On the week days he just sits there on his computer doing god knows what, and on... show more My son is currently 15 years old. I've noticed that he's been withdrawn from everything for a while now. He's a straight A student, but I never see him hang out with any of his friends and he barely comes out of his room. On the week days he just sits there on his computer doing god knows what, and on the weekends he'll do the same thing but with smoking weed involved. He claims that it helps with his depression better than the actual meds he's supposed to take, but I know he's just saying that. He even went as far as to LIE to me and say that his meds were giving him suicidal thoughts. He must think I'm pretty stupid if I'm going to believe that. He just wants to stay with his weed. What should I do?
Update: The medication is for depression and he refused to take them because he says they give him suicidal thoughts, but he's just trying to get me to let him smoke.
Update 2: Suicidal thoughts ISN'T a side effect. How stupid does that sound? Side effects aren't meant to happen, and how does something "accidentally" say to his brain "You should commit suicide". It doesn't work like that, someone has to "program" the medicine to do it, and I'm... show more Suicidal thoughts ISN'T a side effect. How stupid does that sound? Side effects aren't meant to happen, and how does something "accidentally" say to his brain "You should commit suicide". It doesn't work like that, someone has to "program" the medicine to do it, and I'm pretty sure they didn't do that.
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