Salam brothes and sisters, I have a very big problem with my brother and I really need some advice on how to handle this situation.

My parents decided to move my brother(that is 16 years old) to a better school, because he's grades were really bad. However we realised too late that the new school was filled with non-believers, personally I did not think that they would effect him, I was so wrong. To make it short, he got new friends most/all of them non-believers. Slowly did he being to changes, cloths style, music etc. But that's not the problem, he began to smoke and he also got a girl-friend, and it would not wonder me if he drinks and smokes marijuana. Sometimes he don't even come home in the weekends. The thing is that, he thinks that we don't know about his secret affairs, but I do. He cannot even lie because I have proof of everything. how should i confront him, should I tell my family? I just him to drop his new way of life and follow the straight path.