What do historians generally think of race realist ideas? Assuming as a non-historian that it is not a generally accepted theory, my question is: how do I intelligently argue against “race realism” from a historical perspective?

Thanks for answering my clarifying question. I thought about this a bit. I agree with the mods that other disciplines may have more (or at least different) things to say about this. If you want to argue from a perspective within the discipline that he himself is using, than you may need to get him to bone up on his Anthropology and Sociology. However, I do believe that as historians, we have something to say about this. Through research, historians can demonstrate the level of contingency in intellectual movements, like "race realism." If a particular finding about race is, for example, demonstrated to be more about the particular historical conditions behind that finding and less about the subject of the research itself then the finding must be called into question, or "reread" as a function of a particular historical circumstance (rather than data that reflects a biological reality). With that in mind, here are some works that I think address race science in different eras, but do so by showing how tied in it is to historically conditional factors (and thus a pseudo-science). Each of these would be well worth your time. Together I think they present a cogent beginning to an argument about how race science is not real science, and that its findings depend on flawed assumptions borne out of the vicissitudes of modern European history:

Conklin, A. L. (1997). Mission to Civilize: France and West Africa, 1895-1930, Stanford University Press.

-Conklin's look at French colonialism in West Africa touches on race towards the end, demonstrating how as a construct it developed to answer context specific challenges to French rule in contemporary Senegal.

Herzog, D. Sex after Fascism.

-Herzog demonstrates the pervasiveness of race science, recast as the study of sexuality, in post war West Germany.

Matysik, T. (2008). Reforming the Moral Subject: Ethics and Sexuality in Central Europe, 1890 - 1930. Ithaca, Cornell University Press.

-I found this book very tough to read, and in reading my notes I wonder how helpful it would be. But it is a really striking work for showing how things we take for granted about the nature of the human self are historically contingent. Matysik begins with 19th century assumptions about philosophy and the nature of the human self and comes to argue that much of early 20th century perspectives on the individual human were borne out of very specific philosophical theories that have not borne out.

Vick, B. E. (2002). Defining Germany: The 1848 Frankfurt Parliamentarians and National Identity. Cambridge, Harvard University Press.

-Does for political theory and the conception of "ethnicity" and "nation" what Matysik does for "self." Vick looks at nationalism in mid century Germany and shows how it bled into early race theory. Vick is able to show how what constituted this or that "nationalism" was deeply connected to what happened to be the demographic realities of German speaking Europe in the 19th century.

Evans, R. J. (2016). The Pursuit of Power: Europe 1815-1914. New York, Penguin.

-A general book of history (not a monograph). Evans has the best, most accessible book on 19th century Europe. I recommend it in general, but especially for his chapters on the development of race science out of colonialism. Of all the books, this may be the most useful because it may point you in new directions.

I hope this is helpful.