Help 16 year old brother desperate

I have a question about my 16 year old brother. He started dating a girl who had completely changed him she got him to start drinking and smoking and he's failing the 10th grade. He used to be a loving caring person now he is talking back sneaking around talking back and being down right disrespectful. People may say oh he's being a regular 16 year old boy but I know my brother and this is not like him at all. It's getting to wear he's threatening suicide and have anxiety stacks among other things. What can I do to help him? This girl is so wrong for him and I'm afraid he's going down a rabbit hole and he won't be able to get out. I'm also afraid she is going to get him addicted to something other than cigs and drinking