Well, we still don’t know if it really is impossible for anyone to leave SB. Would really anything bad happen, if Regina or Mr. Gold left? We simply can’t tell at this point, so there’s really no way knowing how he got Henry. But in my opinion it’s safe to say he knew exactly who Henry was and he did somehow follow Emma’s childhood/teen years.
Then again I just remembered that we don’t really know if he knew about FTL before Emma got there and time started to flow again …