Would you clarify - "What should I do?"
Are you asking him to halt all contact w/ her or spend less time w/ her?
How did he respond to what you and his mother asked of him so far?
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Hello and Thank You for using Just Answer. As the parent, you need to make sure that you have set the guidelines and rules of the household and that your son understands those rules. If you have already imposed consequences and his behavior has not changed, then you must sit down with him and the girlfriend and tell them that you do not care for over night visits. Also, you may want to contact the girlfriend's parents as well. They may have no idea that she is spending the night over your house.
If your son is 18 already or soon will be, he will be a legal adult. I am not sure if he will be going away to college or not, but he must learn to be respectful of your desires. Sneaking around behind your back and doing things with his girlfriend is not a good sign of open and honest communication between a parent and a child. Just sit down and have a discussion with him about you truly feel and then give your son time to speak. Try to reach some agreements on things that you may still disagree upon.
Have a great day!!!