Seeing his son in oversized shoes reminds the father of what he has lost, and it saddens him.

When the man points out the creek, his son’s first reaction is to ask if drinking it will make him sick.  He says he doesn’t think so.  The man is resigned to the fact that they are at the mercy of nature, and have to drink no matter what and hope it causes no ill effects.  He knows they are lucky if there is any water at all.

Watching the boy, he realizes the boy hardly runs anymore. 

He’d not seen him run in a long time.  Elbows out, flapping along in his outsized tennis shoes.  He stopped and stood watching, biting his lip. (p. 201)

Even though the man does not say, we can infer what he is thinking and feeling.  Seeing the boy doing something all boys do, and something that reminds him of the life they used to live, makes the man thoughtful.  He is relieved that the boy has some spunk again, but everything about the scene is a reminder of what they have lost.  The boy’s outsized shoes are mentioned, because they are symbolic of the depraved conditions and how the man and the boy have to make do.