How much does your DH do around the house? I wonder if this is as much of a DH problem as a DSS problem..? Like father, like son.
If he’s 23, he still has a teenage brain. The brain doesn’t stop developing until the mid to late twenties and the part that controls empathy is one of the last bits to develop 😂 If he’s entitled and selfish, it’s probably down to your DH’s and his DM’s parenting. Maybe you need to spell it out to him like he is a kid, if no one did before.
I don’t agree with him but I can guess his logic. Perhaps all you need is to explain why his logic isn’t actually logical before he becomes a fully fledged selfish, entitled cocklodger.
If he does his own laundry and room, it is no more work for you having him live there than not, is it? He is correct. You need to point out that he benefits from the arrangement more than you (subsidised rent, never needing to cook, less housework than he would living alone or in a house share) whereas you save a little bit on bills but that is offset by having less space. There needs to be some reciprocity.
If he didn’t do family chores when he was a teenager, why should he do them now when he is paying to live there? Which begs the question, why didn’t he do chores? I assume because he wasn’t there full time so you wanted to spend time with him rather than having him clean your home.
Why should he clear up after you and your DC? He shouldn’t but you shouldn’t have to cook and clear up after him. Communal tasks should be split between the 3 adults taking into account that you and your DH are responsible for 4 of the household members and 4/5 of the mess. Cleaning the bathroom weekly sounds very fair.
Does your DH help around the house? If not, he might have a point that if he is expected to do his share, so should your DH… As I said, like father, like son.
If he doesn’t want to clean on his days off, he always has the option that he pays for a cleaner or more rent to cover cleaning.
Or move out.