In "A Worn Path," why is it significant that the grandson cannot speak?

The grandson
does not appear in the story. The fact that he has no voice illustrates his helplessness.
"Having no voice" has a lot of implications and interpretations, some of which may be
quite fitting or irrelevant to this story. For example, politically speaking, having no voice
implies that one has no political power as a voter, democratic citizen, and so on. It could even
imply that one with no political voice is oppressed in some way and/or is treated as a lower
class citizen. Given the racial elements in this story, one could argue that the grandson's lack
of a voice is symbolic on a racial and/or political level. His generation and subsequent
generations will not have "voices" as long as they are treated like second class
citizens. (Note that the people in the town treat Phoenix condescendingly, like a charity
case.) 

He literally has no voice because he had swallowed lye when he was
younger. This damaged his throat. He depends upon his grandmother to get his medicine to sooth
the pain and open his throat. His lack of a voice illustrates how helpless he is. The image of
this helpless boy, with no voice, also fits the symbolism of the story. According to Phoenix, he
waits for her, "holding his mouth open like a little bird." Phoenix's name is symbolic
of the mythological phoenix (Greek myth), a bird reborn again and again of its own ashes. She
continues to make the journey again and again, despite her old age. It is symbolically
consistent that she is like the mythological phoenix (a bird) and the grandson is also like a
"bird."