Why does the main character speak such a strange kind of English?

There are two reasons why the main character, John, speaks
English in such a strange way.

First of all, I think that
the author is trying to sort of trick us at first.  I think he wants to make it seem
like John and his people are some sort of primitive tribe from the past.  They seem a
lot like Indians from the time before European contact and I think that his strange
English is supposed to heighten that feeling in us.

Second,
he speaks strange English because he is actually living in a time far, far in the
future.  Languages change over time just naturally and English (in this story) would
have changed a lot.  After all, it's been so long that people don't even realize that
New York City was a city of people made by people.  So you know that a tremendous amount
of time has passed between the author's time and the time when this story is set. 
Therefore, it is natural that the English spoken by the characters would have
changed.