Hi, we had similar problem at home.
My sister was addicted to hemp. It was hard time back then as when my mom found out she was only 16 years old. My sister was not visiting school, not coming home sometime for day or two...What my mom did was something I will ever remember. She asked her to talk, but in front of the rest of the family (me and my dad). She first explained that we all care about her a lot and that we love her. She explained that whatever problems she has, the hemp is not the answer and it is only a temporarily getaway of the problem. Then she asked her to be honest with us and no matter how hard she feels, to tell us the problem and let us help her.
Mom's voice in that talk was very calm and supportive.
So my advice is to talk to your kid and try be his/hers friend instead of being the person who will kick their ass out and leave them alone in a hard time. The main point of the family is being supportive and loving, and as my mom says - everyone holding together can resolve any problem :)
This was 6 years ago and my sister is now a mother and has a little boy. She is not using hemp anymore and after what happened then our family become even closer.