SS can sleep on a air mattress somewhere.  Not get a room.   That room is for the baby.  The new baby is less important then SS ?  He is not the father to SS.  But the father to your baby..  Don't listen to DH, JUST make that room over for the baby. 

How old will your son be when the new baby will need a room? I say, upgrade baby, who will likely be a toddler, to a day bed with a trundle underneath that SS can pull out and sleep on when he visits. Or, bunk beds for SS and DS (that DS will be able to use with friends when SS isn't around).

It also sounds like your DH isn't handling "losing his son" very well. Has he gone to counseling? It's GOOD for SS to spend time with his BF. It's also GOOD that DH is acting as a mentor. BUT, DH has to take a back seat right now. SS has two parents; DH isn't one of them. He needs to focus on making sure his own kids have space, and he can do that while also letting SS have a place for himself should he happen to stay over.

I wouldn't make a big deal of it for now, but I would start keeping a calendar that shows exactly how often SS spends the night in your home.  If it truly is a handful of times over the course of a year, I don't know how your husband can even argue for his not-son to continue to have his own room at your house when you have kids who could use that room.  I would also point out that babies have a LOT of stuff that could be kept in the baby's bedroom but would likely not fit in a shared room and would end up in the main living areas.  And if the baby ends up being a girl, I think it's a non-starter and it would be a HTDO.  There's no reason that SS can't share with your son on the rare occasions when he sleeps over.

Does this boy keep a lot of stuff at your house?  Do you have a basement or other area that could be converted into a space for him to either sleep or store his stuff?  My two SDs are with us half the time, and SD7 was moved out of her room to make room for our baby.  We turned a basement room into a bedroom for her, although as it's turned out she sleeps upstairs in the bedroom with SD5 and uses her bedroom for her stuff and to play in.   

I don't understand what is wrong with men like this. He would put his two children who live with him full-time in a room together, to accommodate a child who isn't even his, that never spends the night.  Does he want his two kids with you to hate FauxSS?

There is zero logic to his thinking. Please tell him that it's BS and don't tiptoe around it, just insist that your two kids each have their own room. FauxSS can sleep on a coach, an air mattress or the floor when he graces you with his presence.

JUST had this same disagreement 2 weeks ago! We are expecting in April. SD10 is with us 50/50. We have 4 bedrooms upstairs. Currently, SD room, office, playroom, and guest room. 

I had already planned for the office to be converted to the nursery. It's directly over our master, closest to the laundry room and steps from the upstairs bathroom. Its SLIGHTLY larger than SD10 room, but basically the same (same size closet, etc)

SD10 has lived in the same room for 2 and a 1/2 years with no issues. DH wanted to make sure SD was "okay" with keeping her room. He had no logical points other than "the nursery might end up being bigger than SD room." 

This was a hill I was ready to die on since being close to the washer/dryer and bathroom was HUGE for me. I told DH if SD wanted a different room (she has never even mentioned it) then he has the guest room and playroom to choose room, but that's it. 

Call him out on the zero logic and do it anyway. 

Y'know, this might solve itself with no intervention. If this child has been seeing his bf more often (which is a good thing), the day might come when the bf wants to stop your DuH's perceived interference in his relationship with his son ... 

I'm pregnant with my second and we're in a similar situation. My stepdaughter rarely stays over and I want both my kids to have their own rooms. My husband wants to leave SDs room available "just in case" which I think is stupid. I'm just going ahead and converting the room.

When your SS visits the kids can double up somehow depending on the sexes. I think it's ridiculous for a stepkids that's only there a few days a year to have their own shrine of a room 

Here is what i would tell my dh if i were in your shoes.

DH if BM and Johns son (ss bi dads name) wants to come and spend time here in our home we will make a place for him.  ---OP you make this announcment, you dont ask if it is ok.

In the mean time DH our kids will have their rooms. Wanna paint?

**I would start using both bio parents names if this sleeping issue or other matters  come UP again. Example-- HEY DH do you wanna check with bm and JOHN to see if their son can sleep over. Do you think BM and  John will be ok if their son comes over to see the baby.

JMO

Your kids get the rooms. Not to sound harsh but your DH needs to find a way to back away from a kid that is not his kid....and the kid has a dad.  No way on earth would I allow it. 

My brother is 6 years younger than I am.  We shared a room from his age 4 until 8yo.  I was 10 until 14.  

It was no big deal.  Though when we moved we each got our own room and it was time.

Resident kids get room priority regardless of their age.  Visiting kids fit in where there is space.

At this point your DH is being truly an idiot. In your place, i would not have allowed him to continue kissing his ex's ass any longer. How are you not confronting him for even bringing some other woman's kid into your house??