May I ask how old your son is? If he is young, he will develop a routine for answering those kinds of questions. And how you teach him about this will shape both his self-image and his perceptions about other people.

Most people are probably just interested and curious, and aren't thinking about it as such a personal question since usually people can look at each other and identify where someone's parentage originates based on observable physical characteristics. When they can't, they approach it as a puzzle for which they lack the solution. But if you are thinking of it as challenging someone's parentage, or if they take it a step further by insisting that the answer they are given is wrong, then it could easily get sensitive.

It isn't aggressive to ask the question, and it also isn't aggressive to have boundaries. Aggressive is putting someone down, hurting them in some way, or trampling their sensitivities, feelings, rights, etc. Yelling at them might be aggressive. But just saying something like "That's a personal question and I would rather not talk about it" is assertive - just setting a limit and is well within your rights in most circumstances. Making a joke is also an option, but that is just a little bit passive aggressive in that joking about their question might imply that they asked a wrong question without letting them know why. Most people probably will take a joking answer just fine though. Or he can just tell them the truth. "I'm from Kansas." There is nothing wrong with that, but a lot of people are likely to keep asking because they are still interested/curious about identifying the characteristics that had them puzzled. If he just wants to end the conversation, a simple explanation, "I'm black and white," might do the trick. It tells them what they wanted to know and they will probably accept that.

It all really comes down to what is effective in terms of what he can feel good about. He could try different responses, see how people tend to react, and choose what to say to that question based on the results that he prefers. And it might not always be the same. Sometimes he might enjoy saying something funny. Sometimes he might just want to be left alone. Other times maybe he will feel like talking about it (maybe if the person asking is someone he wants to be friends with and feels like he can trust). He should be taught that people don't mean to be rude or inappropriate, but are just curious, and that he can be confident about who he is and about setting limits when he doesn't want to give personal information.