J.C.
This may sound harsh but I had a friend who went through same thing w/her stepson, except this 20 something wasn't in college. After a few years of letting him mooch off of them, bailing him out of all kinds of trouble, she and her husband finally had it out with him and told him that if he wasn't going to get a job, he wasn't going to mooch off of them anymore and life would be hard. His mom did the same thing, because he acted the same no matter who he lived with. And it was. He continued slacking and they stuck to their word, even though it was hard, and basicly he ended up in Job Corps so that he wouldn't be homeless.
It wouldn't have been as bad if he wouldn't have treated them so badly, and guess what...when he comes home from his breaks on Job Corps, they are still reluctant to let him come home b/c now he got married and he AND his wife still treat them badly...dirty up their house and eat their food....He, the son, goes to work everyday w/his dad while he is on his break and tries to find some excuse every day to get out of it...and my friend (the stepmom) has had to take off of work a number of times to go fix some crisis (or make believe) crisis at their house. So now, they aren't even going to let him come back to their house from Job Corps.
I told you all of that to tell you that becasue it was really hard for my friends to take such a hard line with him, but they had no choice because the son refuses to change. Even almost being homeless didn't change him. And, he was a good kid to start off with.
I hope it works out for you. let us know what happens.