Haha, willo, only if you have MY arsey 18yo in return! I have long thought that some kind of teenage exchange scheme would be good, because teens are generally fine with other parents, and we're generally better at knowing what to do with other people's kids! 
My honest advice about the working/contributing thing is that you have not reached your 'bottom line' yet. You tolerate it. You don't like it, but you like it more than the only other alternative within your control, which is throwing him out. And he knows it. 
I'm not offering this advice cos I think I'm a fantastic parent... I'm experienced and battle-scarred, and speaking with the benefit of hindsight...
My own DS had a bone idle period last year. He left school with enough GCSEs to do A levels or another level 3 course, but low confidence, and enrolled on a level 1 course. His attendance dropped from 2.5 days/wk, which was what the college counted as full time, to 1 day/wk over the year, and I got increasingly frustrated and desperate.
Then the course finished in May, and he did nothing constructive between then and Sept. He was worse, it sounds, than your DS... He was taking m-cat/mephedrone as well as skunk, stealing from me, being violent, getting arrested, and hanging out with seriously dodgy people, including dealers and 2 who have since been convicted for manslaughter and murder 
. By this time last year I was utterly desperate and threw him out twice, but there was literally nowhere for him to go except to the people I wanted to keep him from, so I let him come back.
I had a light at the end of my tunnel, so to speak, which was college re-enrollment in Sept. I told him all through the summer him he had until term started to get a job if he didn't want to study, but that I would not support him to do nothing. Then enrollment day came, and he announced he wasn't going, and that he planned to spend his life on the dole.
I knew I could not stand any more. Literally could not stand it - it was making me ill. My bottom line was reached. I told him in the Fri he had until Mon eve to get a job or enroll in college, or I would throw him out. He told me he couldn't possibly get a job that quickly; I told him that might be true, but he had had 4 months' warning, so my deadline stood. He got arrested on the Mon and spent the afternoon in the cells; I told the police I had set him a deadline and asked if they could kindly release him in time to get to college enrollment, or I would have to throw him out. They did. DS protested I couldn't possibly expect him to enroll after a day like that; I insisted I did. He protested all the way to college, but he went...
It was a turning point. DS got onto a course he wanted, and has turned things round: he has worked hard, done well, cleaned up his act, and has been volunteering as well as studying. This summer he will be working there, unpaid, but getting relevant experience and being constructively engaged. :)
That day last Sept, my bottom line was reached. I knew throwing him out would be awful, but it would be less awful than continuing to live the way we had been living the past few months.
I don't think you're at that point yet willo, and your DS knows it. When you decide throwing your DS out is a less awful option than continuing to live the way you are living, you'll be able to be clear with him about that. How long do you think you can stand things the way they are? If not forever, then I suggest you set him a deadline too... And if you're wiser than me, you'll set it for before you make yourself ill...
Meanwhile, if you're not throwing him out today at your bottom line yet, you need to change the way you respond, as I described in my previous post, and look after yourself... :)