Its time to get the SS out of there. Where did he get the idea he could move the GF in? I would tell both of them its time for us to start shopping for an apartment. Choose 5 for them to pick from and get them out of there.
Everything changes when they get a SO its just part of life.

To spread their wings and fly. Mid-20s with full-time job but still living with parents and co-habitating with girlfriend? If he's old enough to make adult decisions, he's old enough to live an adult life.

Your husband is afraid of losing his relationship as a parent to his son. My relationship with my parents got SO much better when I was on my own. They started treating me as an equal, and I did the same. And that means mutual respect, not equal standing. They were/are still my parents.

You are his life-partner, and he has done his job in raising his son. If they want to stay with you, they need to pay rent and participate in the household. If not, they can find a place of their own and act however they want.

So there should be no reason that they cannot find a place of their own together. Is DH expecting them to live in your home indefinitely? That absurd. You both should not feel awkard in your own home. I think he should give them a time frame for moving out, say 2 months (that's plenty of time). If they can't find a place charge them both rent and set rules like a landlord or let SS figure out where he can live. I am sure he has friends/other family members that will let him and GF outstay their welcome.
There is no excuse for that behavior. Your SS may be angry and distance himself even further, but he will also realize how good he had it at home before he chose to mistreat you both.

Here is what I would do.. Start charging rent. Plain and simple.. and that is a base rent that is for his room and a share of the utilities and food.. (on this I would just come up w/an estimate) then I would further add.. Should he choose to have a friend over for the night.. there is an additional fee to that rent. Say.. $20/night. that would cover the cost to fee that ungrateful b*tch.

Now.. this said.. your SS and his GF should find encouragement to move out. To get your DH on board w/this plan. Tell him that a portion of the SS's rent will be set aside for him in an account until he is ready to move out.. then you will give that portion back to SS (kind of like a reward for getting out of your place) If I were to give a portion back.. I would probably say 1/2 to get Hubby to agree.

Sell it to DH as you are trying to get SS to be a more responsible adult... and Adult that learns to pay their bills before spending all his cash on "Fun". He is too old to be spending all his money on nothing and everything.

Oh.. and make sure you tell DH that the 1/2 rent back when he moves out is a SURPRISE and NOT to be mentioned to SS ahead of time.

Since both ss and gf have jobs there is no reason that they should not be either paying rent or in their own place-emphasis on OWN place. How did it come about that gf was allowed to move in? Personally, I would not let any of the kids in my home, bio or step, move in a bf or gf. I think that when they are old enought to make that kind of choice that they are old enought to get their own place.

Think I am paranoid and wrong but look up signs of drug use /abuse and see if it fits SS behavior/changes. Maybe that will wake up DH.
I just counseled a 22 year old last night who is cleaning up her act , but yes drugs, has there been a chnage in weight or appearance along with behavior?

"We don't understand life anymore at 40 then at 20, but we know it and admit it" Jules Renard

So I'm thinking that GF is eating there most of the time, virtually living there. Are these kids paying rent to you and your H?

Your H needs to respect your wishes in your house. I would tell him that SS and his freeloading GF need to get out. My kids will never be allowed to have overnight guests of the opposite sex, until they are on their own (as in out of my house) and in a committed relationship. AND only then when they need to stay here because they don't live close enough- ie holidays or the like.

I am tired of hearing that "times have changed" from when we were raised... ummm....I don't know about you, but the sexual revolution happened the year I was born and my parents didn't let me bring BF's over to spend the night OR spend the night at my BF's when I was over eighteen and still in their house. My dorm in college didn't allow us to have overnight guests of the opposite sex. Just because some parents are too afraid of losing their kids to be parents doesn't mean we all just jump on board.

At least your SS has a job. My suggestion is either of two options. Don't charge him rent but treat him like the child he is still being. No over night guests of the opposite sex. Period.

Or option number two...charge him rent. One amount for him only and double it for GF if she continues to stay and eats meals there. That may just encourage them to find their own place.

Peace, love, and red wine

Great news that your DH stepped up with SS and GF! Of course they may behave even worse toward you now that you (yes YOU!) have made their lives harder. In my experience, even when DH steps up and sets some boundaries with adult skids, I am to blame because I "brainwashed" him.

Did you know that I graduated from "Evil Stepmother Brainwashing School?" With honors!

You deserve a home that is comfortable for you and free from negative energy. You are under no obligation to house and feed a total stranger—especially one who is rude to you. I hope SS and GF find their own place soon and you can reclaim the home you pay for. They deserve the chance to grow up and be adults, and you deserve to be rid of them.