If he is 18, then he is an adult. treat him that way.
If you don't like everything he does, tough ****, he's an adult!
Treat him like you have to treat your other adult friends in order
to even HAVE any!! Since he isn't your son, he isn't your business.
Stay the hell out of it and luxuriate in the "not having to"!!

Yikes. Lotta anger in that post.

You could wind up in a GREAT DEAL of legal trouble if he has been
living there awhile, if you want him out you are required to give
him the same notice you would anyone if they have been paying rent,

I'm not so sure about that. Depending on your state he could be (and
probably would be) what's known as a "licensee." That is, he has permission
to use your property like a guest. Once you tell him to hit the road, he
has to hit the road or he could be arrested for trespass.