I think David's argument is intrinsically not cultural relativism.
As the name suggested, cultural relativism means that there is no "absolute" standard of morality that can be used to all of the different cultures. However, in the comparison between the infant killing convention of Inuit and the moral convention of modern western society, David said that this convention ought not to be adopted by modern western society. Regardless the detail, David has already posed an cultural absolute principle that he can used to determine whether a culture should think something moral or not. Therefore, he cannot be considered as a cultural relativist, though he shows tolerance to a culture with different moral code.