Explain to your son that anyone can be discriminated against by someone of another race, but racism is something different altogether. The “ism” implies a practice, system or philosophy that unfairly bestows favor or privilege upon a specific race, while suppressing others.
If your son lives in the United States he probably isn’t experiencing racism, but the simmering resentment of those subjugated. That would include women of all races and people of color. As long as there’s inequality these resentments will persist.