Another MamaSource writer wrote about this about a week or so ago - almost exactly the same problem. I agree with you, but right now the problem isn't the kid, it's the separate philosophies of you and your husband. You are right that it is a guilt thing, and he just doesn't want to be confrontational. It's time for couples therapy because the other kids will grow up seeing this and just emulate that behavior. You MUST find a common ground on expectations. Maybe, for example, you can back off on the find a job/pay rent issue IF he gets up, helps with chores or the other kids, or has some responsibilities. The license might be good if you don't then feel pressured to buy him a car. Maybe an at-school job between classes that can go toward his tuition or give him some spending money. The fact that he's just hanging around may be laziness, or it may be depression. Does he have friends?
If your husband says the son is an adult, then what adult responsibilities does he have? If he is a kid, then he can be given jobs and chores and be "ordered around". He can't have it both ways. The big household and the high activity level means you can't stay on top of one kid all the time - but you and your husband will have problems with this which will grow and come between you, and also affect the other kids. Get your husband to explain how this hands-off policy is actually HELPING his son - is the son happy? Engaged with others? A part of society? Feeling good about HIMSELF? If not, this system isn't working. Good luck.