If the order says that you pay support to a certain date or time, then you will need to continue paying, but could apply to the court to seek that order be changed.
If it says that you pay support for as long as your son is a child of the marriage, or a child as defined by legislation, then you have an argument to end support when he is 18. Support is payable past age 18 only if a child is unable to withdraw from parental charge due to disability, or other cause such as attendance at school.
So you can stop support when he is 18 if he is not attending school, whether he continues to live with mother or not. If he resumes going to school, the obligation to pay support could resume. This would depend on how long of a gap there is, and the reason for such a gap.
If he is going to school and living away from home, then there is an argument he is not under parental charge, and support could end. But this will depend on the circumstances of why he is living on his own. Courts have said a child can be under parental charge even living away from home, such as a situation where there is conflict with siblings, so the mother gets the child his own space and still pays expenses for him.
You could send a formal letter to her when he is 18 if he moves out, advising that as he is not under parental charge, you will be ending support. If she tries to take you to court to seek an order that you continue payments, you can at least argue to the judge that based on the limited information that she gave you, this was a reasonable course of action. If she gives you or the court more details to show why the son may still be under parental charge, you can at least argue that any order for support should only resume from when she gave you that information.
You are entitled after he is 18 to know details of his education attendance and plan if you are expected to continue paying support.