What is the theme of The Road?

There are quite a few strong themes in The Road worth considering. For me, the theme of paternal love is the most significant. 

Paternal love
The theme of paternal love is ubiquitous given the relationship of the two protagonists. As the man's wife points out before her suicide, "the boy was all that stood between him and death" (25). In other words, the man's thirst for survival is fueled by the love for his son. While the man may expect his own death, he lives in order to seek life for the boy. Unlike his wife in her suicide, the man does not wish to "save" his son from civilization's destruction, rape, murder, and cannibalism by killing him preemptively. To the father, suicide is only an option for the son if he is to be imminently harmed. Perhaps for this purpose he leaves the pistol with the boy whenever he explores a new and potentially unsafe location alone.

The man's love for his son does drive him to ensure his son's survival. The man frequently demonstrates the strength of this love, most obviously in his unflinching decision to shoot and kill the man who threatens the boy's life. Throughout the entire journey, the man does not kill out of malice or for food. He only hurts others (the man who attacks the boy and the thief who takes their cart) when they have threatened the boy's survival. His wariness toward others, which is well justified, seems primarily for the protection of his son.

In less dramatic and violent situations, the man gives his son as many "treats" as he can in such a world--an old can of Coca Cola, a powdered drink mix. As the man lays dying, he tells his son to eat his share of food, instead of keeping it for himself in hopes of regaining his health. These small gifts and sacrifices are strong examples of his paternal love.