I think the idea of David is a nearly perfectly Cultural Relativism.
First David uses two ways to deal with this problem. We use the moral norm in west which is against the action of infanticide. However, he also uses the idea of the tribe side. He is afraid that the resources in the tribe are not enough so he thinks that kind of behavior is acceptable.
In that case, David understands the idea that different culture has a different background, which will cause different kind of opinion towards each event. That is corresponding to the Cultural Relativism.
What is more, I think that he also stands on the tribe side. Although he lives in the west, we can also imagine how people live poorly in the tribe and the tribe with a different moral landscape. He thinks norms differ from place to place. It is wrong for him if he is not able to aware the difference.