He is trying to sound aggressive. "White boy" in another context could be ok. For example, you're describing a caucasian child - "He is a white boy". Referring to an adult as 'boy' is almost always aggressive, especially if you're not a part of their group. Adding a word about race/skin color when it's not needed is also almost always aggressive/rude. This is my perception - as a white man - so, others may have very different perceptions.

A "white boy" is a man or boy that is of the white race. White people are typically fair-skinned and are of European descent. "White boy" is not polite and is aggressive. Don't call people this. I see people avoided this question because it is a touchy subject. Race in America, at least, is a very taboo topic.