Obviously he has an american citizenship, in papers he is 100% American, but his parents are Mexican, he has mexican blood, it is impossible to say that you are 100% from one country if you have blood from a whole different country and race as well.
I was born in Ukraine, that makes me automatically 100% Ukrainian? Obviously not.
I have Mexican citizenship, that makes me 100% Mexican? In the documents yes! But in reality I am not a Mexican at all.
Not only brazilians can do this stuff