Bullying and abuse was something I wanted to ask, but it can be an uncomfortable subject. And someone asked earlier what he had to be depressed about, your poor son, he has been through a lot for someone young, is it any wonder that he has these problems now. He desperately needs to get out on a daily basis. I would speak to him about going back to college. This would help him develop confidence.
This situation is identical to what happen to my ex, he stayed like this until he was 24, living with his family with all the same arguements. Eventually, he went back to college and is now going to university, about 400 miles away from his family! He is, well, happier. But he refused to get help, which is a bonus for your son.
What kind of hobbies does he have?