At the end of the novel, is the boy's rescue too contrived and unrealistic?

What a great question. After reading it three times I have to say that there nothing contrived about this novel. Cormac McCarthy doesn't through around much hope. MaCarthy's post-apocalyptic vision leaves father and son nothing to hang on to. In fact the world has become a nightmare full of cannibals, roving gangs, and wondering derelicts. The fact that the boy is taken in by a semi-intact family simply means there is a chance of survival. The tone of the story doesn't change too much. Nothing promises to be different and the reader is still left with the impression that they all might be better off dead. From what we have experienced in the novel, resources to feed an extra boy are the least this family has to worry about.