Labrador Retrievers

Hi. I'm new to owning a dog. We were given a 3 yr old male Chocolate Lab, and we ALL adore him. We have had him about a month and a half now. Soon after getting him, we switched his food from the Royal brand to IAMS because we just didn't think we could afford the other food. Well, he started having diarrhea right around that time, and he hasn't stopped yet. We took him to the vet, and they put him on Metronidazole and Panacur dewormer. They said he tested negative for worms, but has a bacterial infection similar to us getting food poisoning. We have since switched his food from IAMS to Purina Pro Plan Chicken. We switched much slower this time though. I'm wondering how much longer I should give the medicine before he stops having diarrhea. A lady at the pet store said he could be allergic to chicken and that might be causing some trouble. Please help. I'm worried.
Thanks!
Mom to "Coach"

Ok this is what I want you to do. Quit feeding him dog food for a bit and feed him only white rice and boiled chicken. Itll soak up everything and help settle his tummy. Keep him on that for a week and then start gradually adding his food to the rice/ chicken mix. If the diarrhea doesnt stop youll need to take him back to the vet.

Chicken, lamb, and duck are all easy on their tummies so feel free to try lamb or duck if the chicken doesnt work out well. Good luck.