Help with nephew

I have an adorable almost 2 yr old nephew who’s sweet as can be. The problem is his mom doesn’t take care of him. I got so frustrated the last time we were over there I said we’d just take him in. She was practically pushing him out the door. It’s been 4 days and I’ve really seen how far behind he is. We also had to buy him new clothes and diapers as everything she sent was wayyy too small. I’ve been texting her and she acts like she doesn’t want him back at all. She goes “keep him as long as you want. Theirs no rush”. This is the complete opposite of other parents I know.

The question is what do I do next? The idea of keeping him permanently seems a bit overwhelming but I’d also rather do that than send him back to a home where he’s not wanted or cared for properly. She was shocked that he doesn’t cry and scream all day with us like he does her.

So I guess I have a few questions. One being how do I help him learn to talk? He’s basically not allowed to make any noise at her house so was never given the chance to learn. Here he babbles and giggles all the time

Another big issue is stranger danger. He screams horribly any time a stranger looks at him or hugs me.

And he can’t handle me being on the other side of the room. He freaks out if I’m not within like 5 ft of him.

I’m hoping she just needs some time to reset and figure out her life then can be the parent he deserves since I know she does love him but either way if we are going to try and keep him permanently or if this is just temporary I want to help him work on these things.