My son (17M) won't stop smoking weed.

Some back story: I caught my son Paul with weed last year. His dad and I have been divorced for 14 years. One night when he was out with his dad, I went into his room to find a pencil eraser and smelled weed in his backpack. I looked in it and found a pipe and a plastic container with a few buds in it. I confronted him about it, and we had a heart felt discussion. He begged me not to tell his dad (his dad is a fundamental born again Christian and would have gone nuclear on him). I agreed to not tell him if he would stop smoking and go to therapy. I got him into therapy for a while, he stopped smoking, and everything seemed OK. At that time I made it clear I didn't want him smoking.This year, he's a senior. He had his first girlfriend. They went to Homecoming together, his grades were good, he seemed happy. Well, they broke up a few months ago and he went into a downward spiral. His grades tanked and he was very somber and withdrawn. I tried talking to him about it but he didn't want to tell me anything. Thing is, and this is where I screwed up, before the breakup I knew he was smoking every once in a while behind my back with friends because he would sometimes have that "look" and behave like he was high. It wasn't that often so I overlooked it. I didn't take a very hard line with him and now I feel like it's my fault that things have gotten out of hand. After the breakup, he's been coming home high often. I confronted him again and well, Reddit, my son is has become a full blown stoner. He smokes everyday after school with God knows who, he has no motivation, he doesn't even seem to care if he graduates. I told him the other night he has to stop or I will tell his dad, and he just acted like he didn't care. His personality is different. He just seems to have no emotions anymore. I am scared. He's talked about leaving before when I've taken a hard line with him. His dad is so extreme that I am afraid he may do something drastic that will backfire. His best friend from school ran away from home because his mother was being strict with him and I am afraid he will do the same thing and run.I don't know what to do. I feel like a horrible parent. Like I failed him by not taking a harder line with him. He told me he feels like smoking weed is part of who he is now. He was on track to go to college, his dad and I have money saved to send him to college, and now I am terrified he's going to throw away his shot at a decent education and future over weed. He agreed to go to therapy again, so I got him an appointment but it's not for another week. Should I tell his dad? I am so conflicted right now because I don't know if his dad can be reasonable in a way that will help the situation rather than hurt it. At this point, I've been crying a lot and the last few nights I've been having nightmares. Have any of you ever dealt with this and had a positive outcome? He's becoming so distant. What can I do that won't make things worse? I told him on Thursday he had to stop smoking. He came home from work high last night. Oh, and he recently got back together with the girlfriend so that's no longer an issue. via /r/Parenting http://ift.tt/2iblGcc