I think that some of the most powerful words regarding the meaning that father and son search for in their lives comes from the basic idea that their will to survive reflects the meaning in their shared consciousness. The will to survive and to persevere is where both find meaning to their lives:
An hour later they were on the road. He pushed the cart and both he and the boy carried knapsacks. In the knapsacks were essential things.... Along the shore a burden of dead reeds. Are you okay? he said. The boy nodded. Then they set out along the blacktop in the gunmetal light, shuffling through the ash, each the other's world entire.
There are some elements here from McCarthy's narration that brings out how the boy and father find meaning through living. Consider the barren nature of the world in which they live. There is nothing there except for one another, "Each the other world's entire." This helps to bring out how there is a meaning in living for the other. The fact that both of them carry on, without hesitation or reticence, secure only in the knowledge that they are there for one another is where meaning for father and son exists in knowing that each lives for the other.