okay, why Rex its wearing Argentina (becuase im from argentina and rex knows who will win in Brazil). jajajaj just kidding.
but i have this headcanon where Rex try to have something to relate to his father, he dont remember anything, but he want to make his dad proud and after discovering the truth about his parents he try to learn more about their cultures. and futbol is one, (i think he have lots in common with his mom with all the soup operas and all that). but you might say, why dont mexico? well futbol, like any other sport is a thing between dad & son. thats why he choose argentina. (but he also can go for Mexico and Switzerland, and why not USA, but Argentina is the main one)