Put a spot in your room, where he can sleep when and if need be.
Like a floor futon or a cot or a sleeping pad.
That is what we do... with our kids. It works out well for us. That way they can be near us, (not in our bed) but sleep and if going through any night-time "fears" which do occur and is developmental based.
Kids just simply get scared at night... all alone, in the dark, in their rooms sometimes. No biggie. I did that too as a child. My parents let me. I grew out of it. And sometimes, a child just needs the closeness of their parents. Its normal. And to me, it is not a "bad" "habit."
There was a time when, when I had my 2nd child, my son would be in his crib, in our room, and my daughter would be sleeping on the floor futon in our room... and we'd all be in the room. We all slept well that way.. .and it took away any sleeping "battles" etc.
AND, your eldest child/baby does know on some level that you are pregnant and something is coming up... so they need comforting. Its not easy for them either.... the adjustment of a "baby" etc.
All the best,
Susan