Why is my dog more interested in cat shit than dog food?

We need to get grain-free because of our other dog's allergies, but even before then, this mutt was more interested in trying to steal table scraps and raiding the litter boxes. The only thing that stops him is keeping the hallway door shut, which we can't really do without the cats scratching at it every five minutes. He'll also go into my room to eat the kibble the cats knock out of their bowl, but again, all but refuses to touch the dog bowls in the kitchen. Is there any way to stop this short of rehoming the cats or dog?