Your partner's nephew already has behavioural issues, due to his upbringing. Those issues aren't going to magically disappear, if he comes to live with your partner (regardless if you're living there or not). In fact those behavioural issues will ramp up, boundaries will be tested to the extreme and the nephew is going to need a huge amount of therapy. Your partner will need to give a lot of his time to his nephew, be consistent and show him that he's not only going to be there for him (no matter what) but he loves him.
The nephew needs his own space; his own space can't be a sofa in the lounge. I'd be surprised if SS deem this suitable for a child and it's hardly going to make him feel secure, if a bedroom can't even be provided.
Your partner is massively underestimating the level of support his nephew will need. If your partner doesn't go into this with his eyes fully open, and understanding, it will go pear-shaped very quickly (regardless of you're there or not). This is a man who doesn't have any experience of children, let alone traumatised ones. He's being worryingly naive.
How is your partner going to have the time to devote to a troubled 10 year old and a newborn, plus working??? His time will be taken up by his nephew. Or is he planning for you to be looking after 4 children???
Which would be worse for his nephew.....coming to live with his uncle, and then having to go into care because he can't cope or going straight into care? Neither scenario is great, but it would be far worse to have to go into care, if your partner finds he can't cope.
Your priority has to be your own two children and your unborn baby. You can see the pitfalls, your partner is looking through rose-tinted glasses, and can't.