What are you currently feeding, and did your vet give you any suggestions for what to feed? Is he/she saying it might be a food intolerance causing the liquid poop? Did you have a particular food in mind? Part of the problem with switching foods is that if he's also getting the Metro and Panacur AND you switch food at the same time and things clear up, you won't know what helped, the meds or the food, and you may end up thinking he has a food sensitivity that doesn't exist.
Aside from that, if you go for the switch, I'd probably start with the smallest bag and see how things go. I went through 3 rounds of diarrhea and meds on my younger pup when she came home. Things started out firm though, so when the diarrhea started 2 weeks later, I did the meds and did not switch food even though by the 3rd round the vet said I should switch from a chicken to a fish based food. Repeated tests did not show giardia, the vet said she had probably had colitis. Why? Who knows.
Good luck, I know it's frustrating trying to find the right combo to clear things up.