But why that particular flag?
Because he believes the races should be separated and that blacks should be controlled by whites?
Because it represents Rhodesia (in the one case, nowadays "Zimbabwe") and old South Africa in the other, both represent countries in which subjugation of blacks to a white minority was standard, official practice.
The kid was some kind of an "honor student" which implies he was more knowledgeable than your average ignorant "yout.'"