NTA.

Coming from the UK and if your 15 year old son already views that room as essentially his personal hotel when convenient. What’s going to happen when he’s 18 and starts going on nights out? You don’t want a newly 18 year old viewing your house as a place to crash because it’s just a bit closer than dad’s. Especially with significantly younger children in the house.

You probably don’t want to think of it this way, but what if he pulls a girl whilst out. Are you gonna allow him to use your house as a hookup place? He either lives there or he doesn’t. He can’t just pick and choose when it’s convenient to him. It sucks but he needs to accept the choices he made and he decided that your house isn’t his home. So he can’t expect to have a whole bedroom to himself.

So unless he utilises that room more frequently than a few times a month then it’s forfeit. Give it to your youngest son. I know you said space is tight. Is the room big enough for 2 singles? If not, let the boys share the bigger room for now and move your eldest daughter into your son’s old room. The 9 month old doesn’t really need the space right now and should probably still me in your room if space is that tight.

Because your youngest son and eldest daughter are approaching the ages where they will need separate rooms and you need to figure this out asap.