Have a talk with him and let him know the consequences if it happens again, whether in front of you or if you even hear about it from her when you aren't there. Let him know what privleges will be lost in what order. If you want to be patient then treat him like a child and take away a gaming system or remove the data plan from his phone bill if he's on your plan. Take his door off (it's so easy). Perhaps he should sign a contract with about it and about the consequences. Ultimately, start planning to kick him out. The longer you tolerate that kind of behavior the worse off EVERYONE is. Does he have younger siblings learning this behavior?
There is something happening where these individuals are taught to disrespect and disrespect others with no consequences whatsoever. The result is the most disgusting thing I've ever seen someone do to ANYONE. I can truly think of no better feeling then receiving such disrespect for the simple act of saying “hello” to you. “Hello” is the best response anyone can have when they've been asked for help/ advice. To this child it seems like “Hello” has no value, it's nothing but “Hi Mother!!” How can anyone stand for that? If your child does that, what will you do to stop him? As parents, we have to start thinking about what it is that we want our kids saying and doing, I believe this is the first step in teaching them empathy. It may mean getting in touch with the child through the video apps that your child has.