What should I do?

I like this guy and he has been hanging with the wrong crowd recently. I think I may be falling for him but it hurts my heart knowing that he is doing dumb crap. For instance, he has started a nasty habit of smoking. (Weed, Cigarettes, etc.) I'm worried about him and he has withdrawn a lot from me recently. I don't know if he's ignoring me or if his parents found out about his bad choices. Then there's the fact that he knows that what he is doing hurts me. He says that I don't understand and should try it with him sometime. For some reason he gets possessive over the idea of me doing that stupid crap without him there to protect me. What do you think? Should I not worry about him? Is he worth a second chance with me?