The reason he sounds young is because, in a lot of ways, he is.
Makes sense, and I didn't say I didn't like it.
The bit where he calls Nesa his "mother" is because he reasoned it all out logically - she's doing a lot of the behaviours she's attributing to a mother (in the social sense) and she's the right gender, so she's obviously his mother. The fact that it's one of those things which *isn't* logical, and which she's going to have to explain in social context, is one of the reasons she's busy hiding behind my hindbrain and having a breakdown.
I can't blame her, but yes I saw the logic in it, and I just knew that was coming after she told Sephiroth that he was the only person who knew who his mother in the social sense might be.