You mentioned, that when your daughter moved out to college, her room was converted to a guest room. And now, when it's his son's turn, this is not being accepted by your husband, and why is this even an issue for the ex wife? Did your stepson's therapist find out about the s. ideation, mentioned by your h?
In my opinion, what is good for the goose, needs to also be good for the gander. What's fair is fair.
Hamster gave good guidance, as to trying to sort out what lies behind your h's stance. More importantly, your needs are addressed in all this.