You're in a really tricky situation here, OP.
I wouldn't like the responsibility of choosing between my partners nephew going into care or coming to live with us.
But ultimately, I feel your are being more sensible and level headed about the actual implication of your DHs nephew coming to live with you.
In the nicest way possible, I don't think your DH is thinking things through properly. The practicalities. It seems to be very much "it'll be fine, we'll make it work" with no thinking going into actually HOW that will work.
I didn't rtft but a PP did point out that Social Services wouldn't let a child sleep on a sofa. I don't know if this is true but it sounds right. They'll likely want the nephew to have his own bed/room.
It could very well be that you're assessed and not deemed to be a viable option anyway.
I do know from a friend of a friend that when Social Services are looking at placing with family, the assessments can be quite intense/detailed. It could be they'd say no, regardless.
I'm really sorry, you're in a very sticky situation and all while being pregnant too.
💐