6 mos old golden pup has watery, frequent diarrhea?

We've been to the vet yesterday and he said to wait it out on a diet of Iams low residue and gave us metronidazole to give 2 times a day for a week. He didn't seem too worried b/c Ollie looked good overall. Lat night however he started the frequent, explosive watery diarrhea. He went almost every hr. and is very sleepy and depressed looking. He did eat a bit this morning and was looking for more. He has been drinking a small amnt. of which I added Pedialyte to so he can replenish. Obviously, I will call vet again today but thought I'd get some input first. Anyone know what this is? I brought a stool sample but he didn't check it b/c we were just in last month and had it tested and all was neg. Ollie also has had intermittent loose stools since Feb. Nothing to this extent though. Any ideas?