Why does our mastiff eat our other dog's poop? How do I get him to stop?

I agree that you should look at their food. Buy a protein based, high quality food like Innova, Blue Buffalo, Orijen, California Natural, etc. Just check the ingredients- make sure the first ingredient is meat and not corn or other grains. That should solve any "protein deficiency". You can also make sure the other dog's poop is unappetizing... Put a small amount of pineapple in both dog's food; it will make their poop taste horrible, and they should stop.