Would love to say its easy, and you never know it could be, so try not to stress before you get there - self-fulfilling prophecy and all that. But for what its worth, I don't think you have an unrealistic picture in your head, I just think its something thats going to take time and effort.
Will probably have moved on by the time I have finished writing this but...
My pearls of wisdom continued, from when I went through something very similar to this:
Remember that they are a child and much of what they do/most is not done with malicious intent.
Remember that mum might have been whispering in his ear/you don't know what story he is getting at home and that can be increasing his insecurities.
He may be jealous of your relationship with your son/and other half. He will see you all getting on/working as a unit and feel excluded from that, so the more that he can help you do mundane boring things like choosing DS's outfit, getting nappies/passing the wipes, clearing things away from breakfast, the more he will feel integrated in to everyday life. Bizarrely, once my DSD realised that it she was part of the family, so actually didnt want to go out on day trips and preferred to be at "home" even when we all wanted to go.
Being able to spend time with him and you alone is just as important if you want to create a family unit at home - perhaps one of the things you can do is just put in a standard bed, and just the two of you can spend an afternoon going out and getting things to personalise his bed area and doing that together - its a harmless way of getting more in touch with his likes/dislikes and shows that you care just as much about him as you do your own DS.
The more normalcy you can add the better - letting him find his position comfortably in the unit takes time, but if you engineer things then it makes it harder - not fair weather parenting basically - its good to let them get bored/come up with things to do sometimes.
Be prepared to explain why some things are different - for example the way you deal with a 15 month year old and a 9 year old that are doing something naughty is completely different - you and I know that - but don't expect the 9 year old to understand it. We used to explain it as "You are 9, you know what you are doing is naughty, but DS doesn't know whats wrong/right yet so we have to teach him".
If I was in your position, I would probably use a little white lie when it comes to the bedroom situation - tell him that DS is sleeping with you because he's not been sleeping well recently and you don't want him being disturbed - but reiterate that it is their room, shared - otherwise he may come to expect it to be vacated every time/if things don't get comfortable quickly that is.
I can not emphasize enough how important it is that you and DP are joined up and supportive of each other in this, if there are any cracks at all they will be found and exposed.
Good luck - I'm sure it will be fine - and you will all find your feet in the end.