Metronidazole can cause diarrhea as a side effect. But it may be also working on the infective cause of your dog's illness in the first place, so don't stop it without speaking to your vet.
Meanwhile keep your dog hydrated. If you can find an unflavoured electrolyte drink like Pedialyte that would be great. Plain water would do for a short time but Pedialyte would be better.
You can improvise using some boiled water, a VERY small pinch of pure rock salt or genuine sea salt (no additives like anti-caking agents etc!) and a small amount of honey or maple syrup. Dissolve in the water, cool it, and give him that to drink. That might help his energy and keep his electrolytes up as he will be dehydrated from losing excess fluid through diarrhea.
Try feeding him plain boiled white rice and if that helps, introduce some plain boiled chicken to the rice. Small amounts at first and see if it helps.
But do speak to your vet also.