Our dogs can drink about 10 cups in a day, LP--they're about 50 - 55 pounds on average. Can't remember how Toby compares. A larger dog might drink a couple cups more, a smaller dog a little less. But a gallon seems a bit high.
If he's dehydrated from the loose stools, though, he might be drinking more to compensate.
Just an aside--we use metronidazole a lot for a number of maladies
and one thing we've noticed is that although the drug usually stops diarrhea, the stools never really firm up to normal until after
our dogs are off the metronidazole entirely.