Kate; That all sounds fine except for one thing - telling him he has to leave does not work. What will happen is this - he will start smashing windows, walls etc. and then I will have to call the police. Then he will have a record, lose the job it took him 1 1/2 years to get, and never speak to us again. We already had something similar to this happen when he left to live with his girlfriend. Also - what about the addiction? How does that play into all of this and how do we get him off the computer so he can have a life and move forward.
It is a difficult choice to make. His behavior is such that he is forcing you into either calling the police and having him removed, or letting him stay and you end up supporting him. He has you backed into a corner and it leaves you to be the responsible one since he refuses to be responsible for himself. No one can control another person, especially one that is an adult. And if he has an addiction to the computer, he needs help. But he won't get help and you can't make him since he is not a minor. That leaves you to decide if you are willing to let him manage this on his own or let it become your problem. And if he does act out if you make him leave, that is on him You can't control what he does. You can only control your own situation.
You may also want to consider seeing a counselor to help you cope with the stress of the situation. It can help greatly when you have someone to talk with as you work on getting your son on his own two feet.