Can anyone help me?

Ok. My best friend has recently been drinking and getting high many times. He has been getting "Sh!t faced" many times. I don't know exactly what to do. But it is really effecting him menatlly,health wise, physically,etc. He has become very forgetful. He is sleepy all the time. He's been experiencing mood swings. He has put on so much weight. His judgement has been impared and really doesn't know right from wrong anymore. I think part of it is my fault because me and him had a falling out a couple of months ago and when that happened he started hanging out with this new friend and the more he hung out with him the more he did this stuff. But we went back to being friends and then I discovered he was doing this stuff. I did tell a teacher at school about it, they talked to him about it. He said he'd stop but got back on it almost immediatly. I don't know what to do now. Can anyone help me?