The Dialogue Itself

I ain't gonna 'splain it all to ya today.We recently covered those damned dialogue tags, which definitely have rules.  Now let’s look at dialogue itself, where rules go to die.  A fellow writer posted up this question:
So a teenage boy walks into a cave and says, “There's bones everywhere.” Or does he say, “There are bones everywhere.” I'm not wondering which is correct. I'm wondering which you think a boy would say.
If you want grammatically correct, “There are . . .” is the right wa...