The boy and man have very different reactions to the old man and how to treat him.

The man is suspect of everyone they meet. This is with good reason as humanity has suffocated and most people left have no sense of right or wrong. People are merely animals surviving by killing anyone. The boy is a Christ-figure in the book. He looks for good in people. He still feels empathy in a world that has long forgot what empathy is. The boy wants to help the old man. The father doesn't trust anyone.