if he has both American and South African citizenship, he'd be South African-American. similar to how a white person with both Chinese (idk how citizenship for non-payment ethnic Chinese people works, though) and American citizenship would be Chinese-American, and not Asian-American
"African-American" is an ethnicity designated for black Americans.
sorry brother, but youre wrong on this one. while you are correct that hes not an african-american by the common usage of the term, he absolutely is one by the technical definition. better luck next time