I think ISABELLA27 has a good idea. Move his stuff in the room, like the TV and do the colours like his old room, see if he is comfortable with it.
I have Aspergers and so does my youngest brother.
I'm strongly obsessed with Doctor Who and I had a middle sized room and there was hardly any room for me to sleep because I had so much Doctor Who stuff. So I needed a bigger room just for that. I now have my own flat and there is Doctor Who stuff in EVERY room.
My brother has the smallest room. It has his bed with a pullout desk and some draws. (that his playstaion and tv sit on) That's it. That's all he wants. Like me he has no friends so doesn't need the room for friends coming round.
If your son has an obsession like mine with Doctor Who then you could get loads of that to go in his new room. If not it will probably be best to leave him in the little room and give baby the big room.
If baby turns out NT then they will probably like the bigger room as they will have friends round to play etc