Responsibility for a young family is a lot to carry for an 18-year-old who earns less than US$300 a month, half of which he sends to Villaguay, far to the north of Buenos Aires. It goes to pay the labour on a house he's building a few blocks from his mother.
An adult boy
"He has a clear objective, he is going to devote himself to it," said the Pitbulls' trainer Mariano Carrera, former WBA middleweight champion.
Ahead of the Youth Olympics – a scaled down version of the Games for young athletes being held in Buenos Aires – Brian and his team have spent the last 10 months at Argentina's high performance center.
Whip-thin but hard as nails, he's a superwelterweight at 69 kilos, and has more than 200,000 followers on Instagram. Many are adolescents like him.
"He's an adult boy. He's very responsible, and he can be Olympic Youth Champion," says Carrera.
"I'm relaxed waiting for my moment, I want to be a professional, live boxing and be a world champion," his young charge says without hesitation.
When he steps in the ring this week, Arregui's inspiration will be another Argentine – Brian Castano, the undefeated WBA super welterweight champion.
He likes fighters who go in the ring and don't hold back.
For Arregui, life is like sport: "He who wants to grow has to take the hardest blows."