Why does the man allow his son to hope and have fantasies about life in the south?

The idea of hope for his son is the only justification the man can use to continue the journey. The slim idea of hope is the only thing that separated mother's decision to kill herself from father's decision to move on. The only other option was for father to kill his son and himself. Unfortunately he only had one bullet. Was it prudent to look for hope in a dead world? Perhaps the father's love for his son transcended reality: perhaps the fantasy was the only concept left to comprehend.