Hi Andy. Maybe not what you are looking for, but since you asked for a dialogue, here is my two cents.
You cannot protect Liam from the world and what people will do or say; however, you can give him a sense of what is right and ethical and the foundation and security to express such convictions, regardless of the climate and popularity of his view. I think we, as parents of any child, cannot ignore such comments that will contribute to how our children perceive themselves and the world; it is especially true when raising a minority child or being a member of any non-traditional family. Consistently ignoring such expressions is equivalent to the approval of them. The comments of your neighbor is particularly disconcerting to me for several reasons. First, it perpetuates that idea that anyone not of Western European ancestry is a "foreigner," which is well, laughable. Secondly, if he knew much about US history (and Canadian too, I think), he would know that "those people" contributed to the construction of and maintenance of our society, both literally and figuratively.
Andy, I would never presume to know anything about your neighbors other than what you have shared here and would NEVER try to offend you; however, is there not a small part of you that wonders if they are so open about a distaste of those who do not share their experience or at least make assumptions about them that they would not do the same, in different company, about same sex partnerships.
I think it entirely appropriate even weeks after such a transgression to broach this subject with the adults. Maybe you could tell them that you have been thinking about the exchange in the car and that you were upset or offended. Maybe you could tell them that you do not define and label people in such a manner in your home and that in a continued relationship with Liam you would request the same of them. I think racism is so insidious; often times people, when approached about something they said might deny the implied meaning or say, "well I did not mean . . ." I have always found it most confounding that there is an assumption by some that if you share "whiteness" expression of such views is appropriate or fair game. My sister, as a very successful attorney in the good ole boy system of Texas, with an Eastern Indian husband, has had many such experiences.
You clearly care about them and their great relationship with Liam; it is obvious. We, too, have a diverse group of friends who differ in religion, ethnicity and sexual orientation. If such an infraction occurred with me, about any of these differences, they would never have the pleasure of my company again.
They would not get an explanation either. Liam is so lucky to have you as a mom.