I personally dont see it making a difference. Why did you let the girlfriend move in??
There are homeless shelters. If you cant stomach that, set a time limit for them to save up money for a room or cheap apartment.
Dont waver. They will never believe or respect your words if you do. They will wslk all over you and not change without consequences.
Do they use drugs in your house? Listen to your rules? Why doesnt son at least apply for Disability and pay rent? Are you prepared to also financially and physically take care of the children they are probably going to have?
I personally know I would not allow the girlfriend in my house and, since I expect my kids to act like adults when they are, son would have been gone for me too if he refused to work.
I am probably alone in this, but I dont feel it is helpful or character building to buy them cars or other big toys. To me, they can ride a bike or walk and save up for a used car. Its healthier than driving and better for your bank book unless you are rich. have two kids who were difficult once and the money stopped. Both now support themselves. One still has a difficult personality but he has a good job, house etc. The other is doing great in general. I have a son with autism who works two jobs and has his own apartment on his own dime and with autism anxiety is HUGE but he is fighting it. He does get a small amount of Disability too.
I dont think it is healthy to let adult grown children run our lives. I have been diagnosed with generalized anxiety disorder, major depression and panic disorder...many times. I try and work and do what I have to anyway. Your son is 23. If his anxiety is that bad he can get help from a county clinic with a sliding scale. You may feel badly for him, but he is a man and looks like one to others and society wont excuse his behavior due to anxiety.
Having said all that, you are your own person, not me, and you do what you feel is right and can live with. Just dont forget he is not that cute little boy amymore. He is a grown up adult as is the girlfriend. You dont have to support either of them, unless you want to. I suspect your boyfriend sees these two as lazy adults, which is how most will.
I wish you to sit down maybe with a therapist to decide the best path for yourself. You matter. You deserve a peaceful, quiet life free of grown sons drama, if that is what you desire.