Do you mean your adult step son stays with you for family visits or he comes and cashes out at your place when he has had a night out?
If the former I would say it would be hard from him to stay, although if appropriate your living room sofa might mean he could stay overnight once your new child was familiar with him, in the future and if this might happen. Although your child may find this unsettling. You might want to talk to social workers about it.
If you mean the later, just crashing out at your place after a night out (nothing wrong in that as that is what young people do) but for a young child can be unsettling! As you said DH has a child aged 21+ who lives with his mother but occasionally stays with us in the spare room mainly if he has been out partying with his friends and doesn't want to drive anywhere or get a taxi home.
So if the latte then I would assume he has been out drinking and would agree with raspberry and say I would imagine they would suggest that doesn't happen once you have a little one placed, it could be disruptive for them and upsetting, even once they get to know their big brother. Alcohol and small children aren't a good mix
Remember any child you adopt, even a relatively young one, may have experienced possible people coming and going in their birth home, possibly staying overnight, unusual, possibly scary people in the house at all hours of day and night and possible alcohol and drug abuse in the house with them as a little one.
So having an adult staying over could be unsettling, especially someone asleep on the sofa meaning they can't get in to watch TV or eat breakfast etc could be unsettling.
If you really need a 'spare' room as well as little one's bedroom could you consider moving to a larger home or building an extension? Just an idea, I know it is not always possible. But if you think you will move then doing so before little one arrives is better.
When our birth dd was small we living in a small terraced two bed with a very small garden. We felt we needed more space and moved when she was young, we were lucky to find a nice place owned by an elderly person that needed work. Once we had one child we knew we wanted another and so a third bedroom was needed but it took us another 8 years of fertility treatment followed by adoption assessment before we finally got our second child.
Re referees, can you cultivate some mutual friends, perhaps people with children as these families who can be a useful resource of sharing ideas/passing on toys and just general support can be good for you both. If you have good friends who then get to know your dh then hey would become mutual friends.
Good luck.