At 18 he "legally" is an adult with all of the rights and responsibilities that come with it. You have the right to tell him that if he wants to stay in your house, he has to follow your rules. If he refuses, you can evict him.
This response does not create an attorney-client relationship and is intended for general information purposes only.
Once he turns 18 years of age, he goes from being a child living in your house to a guest in your house. He can do whatever he pleases at his new residence.