Straight: Jeffery is so poor he can only afford dog food.
Jeffery never could afford anything better than dog food.
Alternatively, Jeffery can't afford dishes, either, and eats out of a dog food bowl. And he can't even afford the canned stuff... He subsists entirely on water and kibble. And maybe some Beggin' Strips or Snausages, but only if it's a special occasion.
Justified: Someone provides Jeffery with only dog food to eat.
Alternatively, Jefferey is seen eying a can of dog food out of desperation. However, in the next shot, he's shown to be eating a suspicious looking plate of meat, with the camera panning to show Fido's collar lying on the floor, and the can of dog food lying in the garbage.
Or he's eating cat food.
But dog food sucks anyway.
...But then the camera pans back, and Fido jumps up onto the table: it turns out the plate of meat was a free steak Jefferey had won, and he wants to share it with his dog, instead of making him eat crappy dog food
Alternatively, Jeffrey expresses his digust at the idea when someone jokingly mentions it, and insists that he could never be so poor that he'd eat dog food... And then it's revealed that he eats cat food.
Zig Zagged: Jeffery gets richer, but still keeps eating dog food, because he is a dog turned into human (and that is why he was poor in the first place). Even after discovering that human food is much more tasty, he still opens a can of dog food once in a while - force of habit.
Averted: Despite the fact that Jeffery is poor, he can still afford normal food.
Enforced: "We have poor character, but he is still not tragic enough." "How about getting him on a diet of dog food then?"