Discrimination as a theme is something that is evident throughout the novel in various guises, but perhaps it is most clear in the way that Assef treats Hassan and talks about him. Consider how he presents Hassan in Chapter Five in a way that clearly indicates how discrimination is linked with racism in his mind:
Afghanistan is the land of Pashtuns. It always has been, always will be. We are the true Afghans, the pure Afghans, not this Flat-Nose here. His people pollute our homeland, our wantan. They dirty our blood.
Assef clearly refers to the way that Hassan, because he is not a Pashtun, is something of a corrupting influence that threatens the supposed purity of Afghanistan. As a result, he clearly behaves in a way that expresses his hatred of Hassan and of all who come from the same racial stock. A good thesis statement therefore might be:
Discrimination in The Kite Runner is reflected through the character of Assef and his treatment of Hassan.
This would allow you to use the above quote to support your point and also to look further at the way in which Assef discriminates against both Hassan and also Hassan's son. Hope this helps!