am I being unreasonable?

I'm pregnant with my first child. Boyfriend has a son already who is 12. He's severely disabled, but stays with us overnight  twice a week (boyfriends days off). Then we have him two times after school ontop for tea (goes home at around 6pm). 
We live in a 2 bedroom flat. One room is ours, others is stepsons. 
I said to boyfriend if we don't get a bigger place, stepson is going to have to share when baby gets older. Boyfriend was furious saying it's no good! I said stepson stays with us two days a week, it's ridcuolous him having that room constantly?! 
When stepson stays with us, due to the nature of his disabilities it'll be easier to have baby in with us! But for the rest of the week I want baby to have that room?! Plus, when baby gets older their bed isn't going to fit in our bedroom. 
I know for the first 6 months baby will probably be in our room, but as it gets older it's going to need more room and it makes sense to me for stepsons room to become shared?! 
The council won't rehouse us as stepson is classed as living with his mum. Boyfriends contact is court ordered and is counted as respbite due to being severely disabled,  so unsure how that works for bedroom entiltlemet, as it seems to differ. But our council have said stepson doesn't live with us so we aren't entitled to anything bigger. 
We both work, low paid jobs before anyone starts with the why don't you just get a job! 
I just don't understand boyfriends point of view. I am getting stressed about space..