I think more information is needed.
In posts like this it's easy to jump on the "protect your child".
It's the default position.
However it sounds like other children are involved here.
The crux of the issue is the behaviour of your son and DH.
As an adult does your son contribute to the household? I don't necessarily mean financially (though if working he should to an appropriate degree) but help with housework, treat the house and others within it appropriately?
You mention privacy a lot. Does your son have any sense of respecting boundaries?
You say you have spoken to your son about your DH's concerns and he ignores them?
Are those concerns reasonable?
As house prices rise having adult children in the home is more common and it can be tricky to navigate.
Have you and DH ever talked about "ground rules"? Have those been communicated to your son?
It's way to blame your DH but frankly I think you all need to acknowledge that this is a new era and talk about boundaries and what's reasonable.
Step parents get a shit time often on MN and sometimes it may be deserved, but there are many other times where the parent and child has unreasonable expectations that in a fully bio family just wouldn't be tolerated.
It's really not clear what the situation is here.