This behavior is out of ordinary for him.
This, combined with your statement from your first post:
He is staying with a girl he recently met.
… makes me think that you might be concerned that there might be more going on than just disrespecting your home rules.
If you are responding to an instinct that his behaviors are not typical for him, then you do need to honor that and do what you can to open up those channels of communication. I think you need to find a neutral way/space to talk with your son so that you can both share how you are feeling without feeling attacked.
If you can’t create a neutral space at home where you are able to have difficult discussions, then maybe consider family therapy, where there is a neutral mediator.
Either way, I think you should let him know you’d like to meet this girlfriend and respect his autonomy in choosing her. Immature or not, he is old enough to make his own decisions.
I recognize that 20 is considered an adult age, but I disagree with the hard stance of, “not under my roof…
(so get your own place)…” He’s disrespecting your rules by spending the nights elsewhere, but he’s not bringing his girlfriend to spend the night at your home, so in his mind, he may feel that he
is respecting your house rules.
Not all 20 year olds are mature enough to be on their own, so we, as parents can find ourselves straddling this grey zone of knowing they’ll “sink” if we take that hard line. That’s not good for anyone… Finding a way to give them space to grow (and make mistakes) while having them respect our boundaries gets more complicated as they age because of the compromise factor.
I am one who has drifted from the Catholic Church, (and the result of that was that my entire family stopped as well.) So I’m sure I have a more lax perspective than many folks on these forums - but I can say that once my boys crossed into that young man stage, all our rules had to be re-written. Figuring out how to communicate, (because they’re not children anymore, but it’s hard to shift out of the “old parent-child mode of communication”…), as they cross into adulthood is key.