In The Road how do the generic labels of "the man" and "the boy" impact the way in which the reader relates to them?

As you are reading a story, most often, characters have
names.  Because of this, the only way that a reader can relate to those characters is
through other aspects of their personality, reactions to events, or their physical
appearances.  By using such generalized terms as "the man," and "the boy," that makes it
easier for the reader to imagine themselves in that situation.  Instead of the son being
"Johnny," for example, he's just a boy; there are a lot more boys reading the book than
Johnny's.  So, it makes it easier for the reader to relate personally and imagine
themselves or someone they know in those positions.

Another
possible reason for not naming the characters is that it makes the story more universal;
McCormac is saying that ANY boy, or ANY man in this situation could have this life.  The
themes are more universally applicable.  The Road is less a
specific story of a specific event, and more of a generalized commentary on humanity.  A
message in the book is that humanity has the potential for great evil and great beauty. 
To exemplify that point, he picks a simple father and son to elaborate.  That makes the
message more universal and general--more like a fable or a proverb, and less like a
fictional entertainment story.

Lastly, terms like "the man"
and "the boy" are consistent with the rather dreamlike, distant, aloof tone of the novel
itself.  McCormac has a very unique style in this novel, and that terminology supports
and enhances that vague, distant style.

I hope that those
thoughts help to get you started; good luck!