I wonder if Scooter could have become sensitized over time.
Might be worth a try to find some better food or just cook for him. It would be cheaper than the vet visits and medication.
When Belle died, I did change his food a few times. He has a sensitive stomach and I was trying to find a food that didn't cause his guts to make all those squelchy noises. Right now he is getting Purina food... Beneful weight control mixed with Purina One formulated for older dogs. He gets cooked greens and bits of meat. And whatever he finds in the yard.