S.H.
For us, we have a floor futon in our room. The kids can sleep there if they want or need to. Its fine with us.
My daughter was a co-sleeper, and my son was on and off.
They have their own beds, which they sleep in too... but they often like to sleep with us... on the floor futon that is.
You can make a room/bed for him..... his own. And see how he is about it all... talk to him about it, let him pick out his his own things etc.
Or use the floor mattress in your room as a just in case thing.
For us though, we do not mind if our kids are on the floor futon in our room... we know they will outgrow that in time, and they also sleep in their rooms too. One day they won't even want us around... and it is actually quite fun having them nearby.
But any transition will take time.... and talking to him about it. So allow for the time it will take. Each child adjusts differently.
Just don't make him feel like he is being 'banished' because of your pregnancy/new baby. But if you do transfer him out of your room... make sure you do it with a LOT of lead time, PRIOR to when you give birth... or it will be too much changes for him at one time... which is "stress" for a child.
The important thing, is to have a regular "routine" every night, before bed, or with his naps. Consistently. That it is not about scolding or punishment or negativity... but about it being comfy and cozy and "happy" for the child. Not saying you do scold for bedtime... some do. But for me, I was never about bed being "forced" or negative... but rather about transitioning it in a "happy" way and offering comfort and understanding about it all. Kids do evolve... and it takes time.
All the best,
Susan