How much time has passed since chapter 13? Why does Hosseini juxtapose the images of a man playing football with his son and a pair of kited flying in the park?

I'm not really sure what the exact year was in Chapter 13. Chapter 14 picks up in 2001. Amir does save Sohrab by bringing to America, but only after he is effectively tricked into doing so. This is why it is the act of running the kite at the end that truly redeems Amir; unlike all the other heroic things he does, it is of his own volition and out of the spirit of true selflessness and loyalty. The football scene between the son and father is the Afghan equivilent of the same relationship.