A good thesis for The Road might be about the persistence of human beings in the face of destruction. The thesis might be something like, "Love motivates people to live in the most horrific circumstances."
Although the man and the boy in the novel live in a terrifying post-apocalyptic world, they cling to life and try to survive. Much of the reason the father tries to survive is the boy. He thinks, "He knew only that the child was his warrant. He said: If he is not the word of God God never spoke" (page 5). The boy, for his part, lives for his father. McCarthy writes that "each [is] the other's world entire" (page 6). The question of what makes humans cling to life and want to live, despite the horror around them, is one that applies to McCarthy's book. McCarthy suggests that love and human connection are powerful forces that motivate people to go on, even when the world around them offers very few other motivations such as sufficient food, shelter, and safety.
It depends what your teacher is looking for, or if they have given you specific parameters within which to develop that thesis. One idea for a thesis can be centered around humanity, as the novel provides excellent examples of the ultimate in inhumanity, and the stubborn desire of the characters to retain theirs. Their struggle to keep that which is human within them alive seems to parallel their will to survive physically. Almost as if those two human impulses are intertwined and interdependent.
This is a good question for the discussion board.
One of the things that struck me when I began reading this book is that nowhere does the author explain what has happened to the world and why it happened. Was there a war? Some kind of global natural disaster? Did an asteroid collide with earth? All the plant life is scorched and so dead that snowfall makes trees collapse.
You might consider centering your essay on how damaged the earth and everything on it is. Considering how innocent and trusting the boy seems to be throughout the book, perhaps your thesis could be something like "there is hope even in the bleakest circumstance."
Can you expan a little more on your idea of thesis for this novel