After anonymous calls, police found the child in the box, which was closed "with a padlocked steel chain," in the city of Putla de Guerrero, local prosecutor Victor Alonso Altamirano, told AFP.
The maternal grandfather is facing domestic violence charges, while the child was taken to social services until authorities determine who should have his custody.
Neighbors told police that the grandfather constantly tied his grandson for unknown reasons, the prosecutor said. It was not clear how long the boy was kept in the box.