I’d say “a few years ago my brother ate the whole box of my trix cereal”. I’ve never realised before but “to eat the whole box” could also mean he ate the cardboard box 😂! We say “box of cereal” to mean both the box but also the things inside it in everyday speech, so the most natural way to say this would be to say to eat a “box of cereal”, nobody will think anyone ate the card 😁.