That does sound like a tough situation. And I can understand you wanting to prepare for it. I think it's a good thing, and I can also understand your DH's POV - there is only so much you can prepare for, there will be things you don't know ahead of time & can't control, therefore can't deal with until it happens.

It will likely be very hard for your DSS to understand the situation until he sees it. While it's worth trying to prepare him, I think you've also got to be prepared for him to not 'get it' until it's there in front of him. He's still very young to be trying to grasp what your DH is telling him.

It will be nice for him to have his own room, especially if he gets to help decorate it & be involved in the process. The biggest thing IMO is to give it time. I know you said you understand it's a process, and that's so true. Kids process things at their own pace. Sometimes the only thing that works, the only thing you can do is give it time & love, and consistency. If you are patient & loving with your DSS he will learn that. Kids have a great capacity to love.
I'm not sure how the relationshps with his mother are... but be careful of speaking badly of her to him (not that I am suggesting that you would). I hated hearing my stepmother bag out my mum, although as an adult I can see her POV. As a kid, I didn't want to hear it & didn't understand it.
And remember that kids minds think differently to ours. I remember a lot of my childhood thoughts & it is so interesting to me now as an adult to be able to remember them. I really did have an entirely different outlook & thought process. It's hard to explain but I keep it in mind as much as I can, that my kids will see things very differently to how I do.

I don't know that that is much in the way of advice for you, but hugs to you & I wish you all the best. It's not easy but you seem to be going into it with eyes & heart open, it's a great start