The kid could be an authority higher than Jacob abd MIB? Hence MIB is so scared of him. There's a lot of mentions of this being a "game" between Jacob and MIB, maybe this kid is the referee or rule-maker.
But yeah, when I first saw him I instantly thought "Kid-Jacob". This could also be why MIB is shocked/afraid.
There's rumours a float of people that think the kid is Aaron, but that's unlikely, unless it's Aaron from the future or something. Or Jacob = Aaron from the future? The whole thing could be a huge a massive link of universes/times? Maybe MIB is from a different universe? Confusing stuff.
Oh and there's a 40-second preview of next weeks ep here
that focuses on Claire and she seems to be wondering where her kid is.