So DS has his own room for 6 nights a week, then shares with a half sibling for 1 night.
The toddler has their own room for 6 nights a week and shares with a half sibling for 1 night.
(The 1yr old has it's own room, because it keeps everyone else up in a shared room and no one wants that.)
And the problem, OP is that the bed in which, for one night, DS and other child share a room, is able to be put away so the other child can have the same kind of space that your DS has for the 6 nights at your house. You do realise that you are the one taking umbrage on this, and your DS wouldn't have given it a second thought. You are feeding your own entitlements and feelings into your DS, and then making it dad and wife's fault.
You can encourage your child to think they are being "pushed out" when they actually have the identical set up as the other child, or you can teach your child "you both have your own room for 6 nights, and share for 1, and it makes sense to have a bed that doesn't take up all the room when you're not there, just like you don't have one in your room with no one in it the whole week."
You only want to handle it in the former way, because you are jealous of your ex's relationship and want to make the wife the villain because of it. The latter is the sensible and reasonable explanation, but you're not interested in that.The only person you are hurting is your own child, because of your feelings. Seriously, think on that.