Welcome, SR! Sorry no one seems to have seen this post earlier...
I have not had this exact experience among my progeny, but have with some of my students over the years. Often, personal experience is actually outweighing rational analysis (as it does with most adults!). I've heard complaints that this specific other student did or was xyz, and received abc disparate treatment (perceived to be preferential), accompanied by attempts to "get" school authority through confrontation or entrapment. It does not appear that your DC is on that track, for which I am thankful!
Unfortunately, many adults presenting equity perspectives either do not have a strong grasp of the data and logic behind them, or deemphasize them in favor of relational, empathetic approaches (nothing wrong with those intrinsically, of course, but they don't communicate equally effectively to all listeners, nor are they a complete picture). Consequently, when your DC challenges them on the reasoning, they are ill-equipped to respond to his satisfaction.
My suggestion would be to look at some data on metrics regarding disproportionality in schools (which is where he spends most of his time right now) or other aspects of society. You know your child best, so you will have a better idea of what kind of data might feel most relevant to him. But if you do a quick google search for say, racial disproportionality in school discipline, and focus on broadly reputable sources, like IES (Institute of Education Sciences:
https://ies.ed.gov), NIMH or NIH, quite a bit of scholarly work should come up about the differences in how schools perceive objectively identical behavior by poverty and race, how they discipline it, and what the long-term consequences to the children involved are on multiple dimensions (access to instruction, grades, dropout, etc--all previously linked to probability of imprisonment, lifetime earnings, and death). You may find that bringing these data to his attention will direct his fairness/justice-seeking differently. Though he may start calling his teachers out on disparate treatment of students in another way...plenty of school personnel (and other humans) articulate equity-inspired ideas, but practice otherwise.
Politics has to do with how we choose to respond to and shape the society we live in. These are simply observations about that world.
