So sorry your little guy is going through this, it must be so stressful not knowing what's causing it.
Could you describe the diarrhea? Is it watery? Is there mucus or blood? Here it describes where the problem can be coming from:
Dog Diarrhea Treatments, Causes, and More
I'm guessing the metronidazole was for possible giardia? How long was he on it? Sometimes the met/flagyl can cause diarrhea. In that case, a different antibiotic must be given.
An appt w/a specialist varies, but in the long run, you'll actually be saving $ or it will be worth it bc a specialist can get to the bottom of the problem. It might be expensive, but going to different vets multiple x's w/no real answers can get more expensive, also frustrating.