I would like to offer that he may be doing more than just smoking pot if his grades are that bad, unless he has always been a bad student. Often the kids will cop to pot, but not tell the entire story. When my daughter was using drugs, she was busted twice for pot and we just found pot and the equipment that goes with pot in her room so we thought that pot was all she did and we were semi-relieved about that. If we had known the full extent of her drug use, we would have freaked out since drugs just wasn't a part of our lives....ever. Not even when others used to do drugs in my high school. Don't get me wrong about pot. I think it is very dangerous to many people and it definitely is a Gateway drug and there is often stuff put into the pot...I am very against it.
Since Daughter quit, we learned she had also been doing meth, ecstascy, ADHD drugs (the kids crush them in pillcrushers and snort them alone or with cocaine), and she even tried heroin, which is making a big comeback. I don't want to scare you, but I do want you to be aware that he MAY just be smoking pot, but this also could be the tip of the iceberg. Beware of weird sleeping hours, weight loss, moodswings, over-the-top anger, friends who also advocate drug use, stolen money (for drugs) and general poor choices.
My daughter started smoking pot first. She was twelve. I didn't realize that it is less common to quit if you start early, but that makes sense. Still, in my daughter's case, she did quit at around nineteen. She has since been clean and we have learned a lot about her drug use and the drug use of other kids, at least as she knew it back then (she is now almost twenty-nine).
I think your son should graduate too. After that, he is 18, your house...your rules. I am not sure what I'd do if my child were smoking pot but also being respectful at home and doing well in other areas of life. I certainly would ban smoking this or keeping any stuff in the house. If your son is really giving you a hard time, I think you have no choice but to show him the door at 18. And I do think you need to tell him you won't allow him to do anything illegal in your home, whether or not he thinks it's ok. Does he have a job? He is old enough. We cut off our daughter's money supply as soon as we saw that she was smoking cigarettes (she has even quit cigarettes). After that, she had to put up with minimal stuff until she turned sixteen. Then she DID work because she got no allowance. It was helpful in the long term to make her get a job...she has a really good work ethic now.
Does your son has realistic plans for his life after high school if he graduates?
I agree that you should go to Al-Anon or Narc-Anon. They REALLY helped me when I felt hopeless and alone.