Yeah, if he went to school there from about the age of six or seven, he'd talk that way.
I think he must be her son. And maybe he's trying to reconnect with her the same way Sydney has been. So he must feel that he has a lot in common with Syd, and if he knows she's his sister and can't tell her, that must be hard...I'm starting to feel kinda bad for Sark. He's probably always wanted a normal family just as much as Syd has.