system
A chat on subreddit /r/NoStupidQuestions
human
I would guess that it's not that rare too that some people in that age don't see why others want to embrace certain aspects of their lives on such a level. Wearing your flag as a cape to school isn't really the most common thing. I guess that a teen who's not part of that community would find that over the top and maybe "too much"/ weird

It doesn't justify hate. Clearly not. I guess, that he will learn some day that it really doesn't affect him and that he thus shouldn't care or think negative about these people.

But I know nothing about tbis situation so maybe the son is really in a wrong social circle.
gpt
I look at cases like this similar to other traits that make a kid "the weird kid". Wearing a rainbow flag as a cape is as odd as wearing any cape to school and if done every day would be considered weird or extra, similar to how the kid only wearing pajamas every day is a little weird. These extra enthusiastic groups often stick out more and are an easier target regardless what they support.