Many people here use Fromm. I use Taste of the Wild. It seems that I have read several times, that people's pup have had that problem with Merrick food for some reason.
The Merrick may not agree with him. When you switch food, do it gradually,mixing a little of the new,with the old, and gradually increasing the new.You are very lucky that your Bowser survived Parvo. To help firm up his stools, you could give him about a tablespoon of pumpkin-just plain,not pie mix,with his food. Hope he checks out okay at the vet!