The reason I asked, "what kind of disrespect?" is because often, we need to be specific in our definitions for our kids. "Disrespect" can be so subjective.
In the absence of a clearly defined disorder, you could be looking at drug problems, or simply someone who is just plain angry and doesn't know how to deal with it. A sense of entitlement coupled with resentment would bring what you describe. You can't fix it by dictating to him because that is not what he respects. You need to command respect, not demand it. yes, you have a right to be respected in your own home (and for your possessions as well as those of others, to be respected in your own home). But he clearly doesn't get the message the way you are sending it.
A book we recommend here a lot is "The Explosive Child" by Ross Greene. OK, your son is no longer a child. These methods still work. I've used them on education departments, on shopkeepers, on all sorts of people. It's collaborative problem solving. Your son is an adult; you need to respect him, even while he is disrespecting you. Now, that does not mean you have to become a doormat - oh dearie me, no. But a classic (short) example - we had a fight with difficult child 3 last night. husband walked past a crowded table and something fell off. He said as he passed, "difficult child 3, please pick that up for me. It only fell down because of your stuff there."
OK, two things wring with that - difficult child 3 didn't see anything drop, and he thought it was a general instruction to tidy up bis stuff - too general an instruction. Next thing wrong - difficult child 3 was totally absorbed in something else and simply did not hear his father.
Next - I walked out, stepped on the object and broke it. It was husband's iPod charger. So who was in the wrong?
If we focus on guilt and blame, we can lay blame in all directions. However, I then got angry with difficult child 3 for not seeing something on the floor and picking it up. Now, I should have known better - difficult child 3 really doesn't notice. If he had picked it up, it would have been a red letter day, worthy of announcement to the skies. So by chiding difficult child 3, I started a fight form which there could be no positive outcome. Nothing learned, nothing changed. But now difficult child 3 was angry and feeling picked on, because he hadn't knocked the thing down and he hadn't stepped on it.
The way I spoke to difficult child 3 - it was disrespectful. But I was angry. I could have handled it better. But we do this often as parents. Also, difficult child 3 is 16, your son is older.
However, while difficult child 3 was shouting at us, we WERE able to say to him, "We are not shouting at you. Please do not shout at us."
difficult child 3 went on to say, "I'm only shouting at you because you shouted at me first."
Now, I currently have laryngitis - shouting is a physical impossibility! It was a matter of difficult child 3's perceptions - although really, it wasn't fair of me to chide him, his conscience was screaming loudly at him and that was what he was responding to. As I pointed out that I could not have shouted, he got a sheepish grin and realised I was right.
You model the behaviour you want from your child (or the person in question).
That said - there is a lot that he your son doing that would be a dealbreaker in most homes. You have two options - kick him out (ie remove the offender) or lock up your belongings (secure them form the offender). Both methods are barrier methods - putting a barrier between the offender and what he is damaging/stealing. It's much the same as putting up a childproof gate at the top of the stairs when you have as toddler learning to walk. You can't trust the toddler to not fall and be injured, so you put up a barrier. When the kid is 10 years old, you don't expect to need the barrier.
Sometimes we need strategies in place even when "at their age" we don't expect we should. And sometimes we have a child who is wayward despite all our efforts.
Something that is screaming at me about this right now - he is adopted. There has been some discussion on this in General, other parents have been having similar problems with an adopted child kicking over the traces and a few parent members have posted their own adoption experiences. Have a look, it might give you some perspective.
You understandably love your son, but probably don't like him much right now. You are right to try to make him accountable - this is what being an adult is about. But you need to be very clear and concrete in your explanation, as well as completely consistent. The car - I would have got it out of impound and then sold it to pay for the fees. Any time you haver an agreement with him, put it in writing and you both sign it. The car - it should have been in writing. OK, 20:20 hindsight. But be clear, be specific, be concrete and keep your cool. Stay quiet and calm even if he is shrieking at you - there should only be one person behaving badly, and that is him. It is not weakness to stay quiet and calm, it is the ultimate in strength. It also give you the most powerful position in the argument - "I do not need to shout. What I have to say is so important, the words alone are strong enough. If you choose to not hear then I will put it in writing. But these are the rules - everybody who lives under this roof complies with the same rules."
Stick around here, get your wife to lurk here or post here too. It really helped me and husband get on the same page far more effectively 9and we thought we already were as effective a team as possible - we upped the ante!)
Marg