Please can I say something in this thread.
I've had a lot of experience with my own "failure to launch" 20-something and I'd beg the OP to be firm (not being a dogsbody) but also patient.
My son was like this for years, he dropped out of uni 3 times, he had no friends, he was virtually unemployable despite never having got into trouble with the law, or messed with drugs. Though he refused treatment at the time he now accepts that he went through years of clinical depression and I'd say it was possible the OP's stepson is in a similar place. I feel for you, but also for your husband and in particular his son who may be hating himself inside.
It's really hard for people who have well-adjusted, self-starter ACs to grasp what it feels like to see your own child simply go into a shell for months and years, living an isolated life, never coming out of their room except for meals (if you're lucky) and contributing very little or nothing to the running of the home. Many people understandably say things like "throw him out", "make rules", "make him pay rent" but have no suggestions as to how to make this happen.
The best you can do sometimes is to keep a safe space for them to live in, make sure they have food, and keep encouraging them to get involved with anything that might help but bear in mind nothing much will work that they have not initiated themselves: it's the old "the person has to want to change" problem.
Having said that I'd suggest you identify a small number - maybe just one or two - specific jobs in the home which he has to do at specific times. OK so he won't clean his own room but he could be responsible for vacuuming the living room every Friday afternoon, for example. Hold him to it. Thank him graciously when he does it.