What do you mean? Skepticism about whether morality exists? He doesn't have to do that, because such a view is publicly anathema. Besides, it is a view concerning positive right, not natural law. Rawls himself was a radical skeptic in the sense that he was a legal positivist. Positivism, in a general respect, is the same thing as moral relativism. They just don't advertise it that way.
Aristotle himself did not bother to defend even the view of natural right, considering that one should despise such ones as who deny it. So far as I know, the most serious attempts in this direction are found amongst the Germans, though, stemming from Descartes and Spinoza's work, deepened in Kant and Hegel, abandoned with Nietzsche, and made clean and palatable again for the universities by Simmel and Weber, with their fact/value talk. It seems to me, such ones as Chomsky simply deny the sense of "skepticism" (though, they themselves are radical skeptics). As though it couldn't possibly exist in their purest of "rational" worlds. Is that exaggeration?
Generally speaking, one might say, there are people like Rawls, who have had a strong effect, and are known to us, because they have been taken up by the living jurists, and treated as viable. And there are those who are famous only amoungst the theorists, as they are called. But that distinction is almost coextensive with the American age, so to say, post WWII. And never was so in former times. Who would have made such theories in the past, Catholics! But, what does one say of them now, "dogmatists"! As though any reasoned argument, however persuasive and sound, must be wrong, because it is also connected to the Pope, and approved by the Pope? A genetic fallacy!
This may be unintelligible as it stands, the issue is that Rawls speaks of a "purely political" concept of Justice. That is where he conceals the positivism. And, seems to be against the stricter postivist, such as the modern Logical Posativists. But, Logical Positivism is something wholly different form legal positivism, and views of living natural rights, the current view of the current society (i.e., relativism).