I have a 12 year old yorkie terrier mix who is pooping like every 2-3 hours - even…

I have a 12 year old yorkie terrier mix who is pooping like every 2-3 hours - even through the night. It is soft, but not diarrhea. At first, because it sort of looked like diarrhea, we started him on a bland diet, but that didn't seem to work. We then gave him a dose of metronidazole for 2 days - that seemed to help for a day, but again, stopped being effective so we stopped it. It has been happening for about 2 weeks, the majority of the poop size is much smaller than usual (round like a nickel vs. a log) but this isn't consistent. This round nickel sized poop isn't always what he evacuates...he will poop a normal log sized soft poop every now and then. There is no mucus or blood, he is definitely straining. He eats Purina Pro Plan sensitive skin and stomach dry dog food, salmon and is eating normal. He isn't losing weight and behaves normal.he was on a Purina Pro Plan Veterinary Supplements FortiFlora Dog Probiotic up until 2 days ago because we weren't sure that was helping anymore. He does squat for quite a while and repeatedly reposition/squat several times before that little nickel-sized piece finally comes out, but for as long as we've had him (10.5 years of his 12 year life), we've called him the "poop-walker" because he always squats and walks around in a squat position while pooping. He is definitely taking taking much longer to poop and the poop-walk can take up to like 5 minutes.