I know this is not a debate, but I would like to pick a small nit with you Zoe. I agree with everything you said except #3. My point from earlier is that you have to teach him that this kind of behavior is not acceptable in the real world. In the end, he will not continue to dirty your house for very many more years no matter what happens. The goal is to raise a man who you can be proud of after he has left you. That means teaching him what is and what is not his, and the kinds of behaviors he has to practice to keep them.
I am not an advocate of giving away his things as a first step. I would recomend packing them up into the attic for a couple months. Taking more and more away until his behavior improves. If you start with something he really likes, BTW, you won’t have to take much else. Then allow him to earn these things back over time. But only if he can pass that time with 0 (or very very few) infractions.
The point is that he does not in fact own any of the things that are “his”. Your job as a parent is to teach him how to aquire the things he wants. You don’t want him learning from the police that he can’t simply take things because he is too lazy to earn it the right way.
Think about it this way. Eventually the world is going to impose its rule on him. If he keeps filthy habits, he will live in filth. He will not be able to keep anything nice, or for long. You cannot protect him from this. Its just the way the world works. You can, however, give him the opportunity to learn better habits before he has to practice them for real. That is, you can take away everything he treasures without taking away the real necessities (like food and shelter).
IMHO that is the purpose of childhood. To learn the way to behave as an adult before you have to do it “for real”.