I see both sides, if your 18 year old son is purposely leaving crap all over the place and expecting you and your husband (otherwise known as the magic cleaning fairy) to pick it all up, then to be honest I can see why it is causing problems, if he's then being disrespectful when asked to do the bare minimum, (as in just cleaning away after himself so not leaving a mess for other people to deal with) then your husband isn't wrong.
I've got an 18 year old in the family who lives with his mum and dad and he does absolutely nothing around the house, he will purposely leave things for others to do, even when asked 3 or 4 times because he "knows they'll take care of it eventually", he's actually a lovely lad in all other ways, very much like you describe but he definitely sees any sort of domestic chores as not his responsibility or his problem, even when it's a mess he's made, if he's not yelled at or nagged he literally contributes nothing, I find it incredibly disrespectful.
I think the step dad thing is a bit of a red herring sometimes, if you think that these problems would still be the same if he was his actual dad, then it's needs removing from the equation, I sometimes think in situations like this, one parent feels like they have to lay down the law because they feel the other is too soft and makes excuses all the time, or they are being taken advantage of, and the child then plays on it saying 'he's picking on me etc', the soft parent thinks the other parent is too harsh, so bites back at that parent.. the child then gets what they want which is to do absolutely fuck all and get away with it.
At the end of the day your son is now an adult and should be contributing in the household.. his room is his domain, so IMO that should be off limits, but everything else in the house is his responsibility to not leave it messy for others to clean up, it's basic human manners, and he certainly shouldn't be disrespecting your husband, it's like he's still in child mode expecting his parents to do everything for him.