It is possible to make the argument that both of them are heroes. The man is the driving force behind the two at the beginning, he is able to control his fear and reassure the boy. The reader sees more of his thought process and understands the obstacles he has overcome to go as far as they have. He also shows great resourcefulness and resolve in the face of horrible situations and odds.
The boy too grows in ability and responsibility throughout the story, certainly making the case that he is the hero, particularly at the close of the story when the man has lost his ability to keep moving and accepts his imminent death.