I can think of two possibilities:

1. The "one drop rule" which has been used in this country for its entire existence, which basically went that if a person had even a drop of African blood, they were black.

2. He identifies as black.

I'm not sure how he identifies, actually, but I think people should get to determine that for themselves and other people should respect it. If someone wants to be called black or someone wants to be called biracial, I just think it's respectful to use whatever they prefer.