My 17 1/2 year-old son confessed to me that he smokes pot 3-4 times a week, and has for almost 2 years now.?

I never did try it myself and am naive about the effects this may have on him. His grades are decent, he is a starter on his varsity soccer and baseball teams, he was voted in this week as the one male junior attendant in the "Swing Swirl" dance, in all other areas he behaves responsibly, is dependable and shows a great deal of common sense. I am just in shock angry, and afraid. He is tells me he is, after having done much research, convinced that it's harmless fun and society overreacts and unnecessarily promotes a negative stigma. He emailed me a VERY long list of websites that (of course) ert the safety of marijuana, but even were he to convinced me it's not as unsafe as I previously thought, it is still illegal and can jeopardize him in that way. He swears he has no interest in trying other drugs.I am very concerned about him and would like to hear from qualified and knowledgeable people what they think, as well as any FACTS they have about such regular pot use. Thank you so muc