NTA. No advice to give really but I have personal experience that maybe your son or his gf would find interesting and maybe add some perspective.

My ex boyfriend and I were together for 3 years. We had a beautiful huge rented home together and gave it up bc the landlords were shit (we reported rat, plumbing, and light problems that were not addressed for months, and then some).

We decided to move into his parents house for a brief amount of time before finding another home. 3 months max. It was during the summer and they had a pool so I couldn't complain.

His parents were so cool with it and I'm super appreciative but I didnt want go overstay my welcome so I was working extra hours and constantly searching for a nice place for us to exceed 3 months.

Little did I know that he was not doing the same. His mother was doing his laundry, cooking, cleaning for him, and he was not planning on leaving bc he had it so good.

I didnt realize this was going on bc I was working so much and we had opposite schedules. I realized he didnt have the same goals in mind and I went out and found an apartment on my own, after 5 months of being at his parents.

Long story short, he was hurt I did this but when I got the apartment he expected to just move in. Which I wasn't having. I worked hard to afford this place and he apparently didnt have any money (he got paid more than me and didnt pay rent to his parents so where did the money go?)

We ended up breaking up and it sucked, it was on good terms because we didnt want to resent eachother for our different goals but I honestly see this happening to your son. His gf sounds like me and he sounds like he is not going anywhere. Dont feel shame for putting your foot down. My exes parents babied him and he lost me and hes still living with his parents, 8 months after we broke up.

P.s. if you're interested, I am now in school full time with a loving bf and a great job. Glad I separated.