How hard this must be for you and how sad your son must feel. My son is also 20 and has such a low self opinion of himself that he wasn't eating, showering, washing his clothes, cleaning his room, etc. He was also using drugs, and when he wouldn't look for a job or look at community college, we kicked him out, but told him if he went to rehab and actively participated, we would consider letting him come back home....which he did. I don't know if your son is using drugs, but my son had gotten so skinny he was almost see through. He was gaunt in his face and never ever slept well, if at all. He was raging all the time. He had become a meth addict.
When he came back home, I told him that I loved him, but that it was unacceptable to treat me the way he was treating me, and that if he ever treated me that way again, I would put him out of the house again. Luckily, he hasn't behaved that way since. He has his hard days because he deals with cravings that are difficult to fight, but on a whole he is doing much better. He tried counseling after rehab but never really connected with anyone well.
I also stopped giving him money, and I will not support his smoking habit. I put gas in his car every once in a while, but he has to work for it. He hates not having any money, and has been working hard to try to find a new job.
Right now my goal is to try to inspire him to be a better person and to work on himself. I have been trying to lovingly disengage from him, and I think it's working. It's not perfect because I have a hard time saying no completely to him, but I keep trying.
Since your son has a job, is going to college and is living out of your house, I think just listening to him and letting him know you care about him is good enough. I think the whole "I'm going to live in my car" thing is just to get a rise out of you. My son did that, and I didn't allow myself to feed into it. I wish I could offer some better ideas, but we're still a work in progress ourselves. I can offer you sympathy and hug, though.