Oh God, I am so very sorry you got talked into putting your dog on Metronizadole even with a negative stool - this was not only unnecessary but that drug is not without side effects and its own harm. I have to admit up front that drugs are a last resort in my home for my pets - only after I have exhausted all natural remedies without risk. We turn way to quickly to a harsh drug for a "quick fix" to things. Please remember that most drugs, except a specific antibiotic targeting a bacteria, suppress symptoms only, rather than cure the heart of the issue. Going after diarrhea will not help you, since you want to find the issue, not treat the symptom.
I think it is worth mentioning here that diarrhea is a symptom, not a problem per se and can arise for a host of reasons. It can be a parasite (coccidia or giardia most common), it can be a bacterial infection of some kind, it can be a food intolerance, it can be too much or too little of something causing loose stool, stress, IBS or IBD, colitis, or gastritis or, very common in giant breeds, a gastro-intestinal imbalance whereby there is not enough acidity and enzymes to process everything properly. Tracking when the diarrhea occurs, its substance, whether it is after meals, on an empty stomach, at certain times of day, when traveling, when home sleeping etc. is important to reigning it in.
Let us know how he is doing and whether the diarrhea is resolved. Given that you said there were some "urgent" cases, it is less likely that it was too much or too little of something. Urgency indicates the colon is trying to expel quickly. I would certainly add a digestive enzyme to his food to help him break down everything - this is good practice even without a problem since why make the body work harder than it has too. If you are feeding cooked or processed food, then a digestive enzyme is absolutely in order. You may want to also add a probiotic, to help cleanse his system, such as Garden of Life. If it is a parasite, there are many natueral remedies that address coccicidia and the like very effectively. I rid a case of it in my home with olive leaf, parsley, black walnut tincture, and probiotic.
Good luck!