OP, I can hardly imagine the instability of that child is anything that any normal parent would want for their child.
A mother pregnant after 4 months, and two new step siblings is a mountain of confusion for any child, to deny it is frankly ridiculous.
Of course he is clinging to the calm of his fathers house and the idea of yet another new child does not appeal to him at all.
This is his truth.
Denying the reality of the confusion this child must be feeling, will not help you.
I would seek professional advice as to how to reassure him best.
It might save you a lot of drama.
It certainly doesn't read as stable, it reads very much like a Jeremy Kyle episode.
Try and save yourself some by figuring out how best to make this ok for this obviously very confused little boy, before any baby appears.
Believe me when you have a baby, the very last thing you would want is for your child to end up as confused as that little fellow in a similar situation.
Good luck.