He's probably on the spectrum - smart, but socially inept. If you want to, you can address that with him, and say: "I can see you're intelligent and above average, even for a German high schooler. But socially, I'd like to remind you of the rules of etiquette. It's not done, here or in any other country, to repeatedly criticize people. You've already told me how American students are behind academically multiple times, so I don't need to hear this anymore. And going forward, applying rules of social etiquette will help you with your professional and private relationships."
Cast it in the form of "social rules" and in light of his future interests, and he will understand it better. I have close relatives on the spectrum - being direct but neutral helps them a lot.
FYI... I lived in the UK, France and Germany as a child, and went to high school in a private international school in Paris. The average American student IS behind the average continental European student!!! It's true

But it doesn't mean it should be said.
My kids went to/still attend good public high schools, and they were or still are in all AP/advanced tracks, which puts them below my high school level in writing and the Humanities, but higher in STEM and math.