I noticed that too. There are apostrophes for contractions like "I'm" but not for won't and don't. I think maybe it's to set the tone, like he was too tired to put them in, or the man is to tired to say them. I think the book is beautifully written, somehow, even though the story is sad and their journey seems hopeless. It's like every time I think it's the end for them, they find hope. Maybe we are supposed to feel their hopelessness so we can feel the relief when they find more food and other things.
I've almost finished the book already, and I realized that there really aren't any spoilers save how it ends (probably). The entire book is the same so far, and I have a hard time stopping myself from just reading the whole thing. I'm afraid I may miss something, but the writing is very simple so I don't think I have missed anything. It's like I'm on this journey with them, and after I stop reading I can't stop thinking about them.