Something I have noticed is if the dog gets too much food stools will be firm at first and then kinda soft at the end. Any time I see this I cut back on food just a little. It is like a balancing act to find the right food and amount for these guys. Any different treats or snacks can also cause soft stools,so introduced only one new thing at a time for a couple of weeks till you see if it affects him. Is Charlie's entire stools formed but soft,mushy pile or very thin? If they have total soft stools for a few days sometimes their rear can become sorta irritated which will make them drag bottom on floor as they are kinda itchy. Or if it is very thin a little can stay on them and cause irritation. I have washed rears with warm cloths or baby wipes before especially with short haired breeds when going through bouts of diarrhea.
Did they check for worms at the vet? Did he drag bottom before he had soft stools? Each time they have a hard firm stool they will express their anal glands a little naturally,so when soft this does not happen. Has he had soft stools for several days or just a day? Has his stools been normal and firm any since he has started the new food? how long has he been on it now? I am a huge poop watcher as I feel this is the way to let you know what is going on inside and if everything is agreeing with them. Sometimes when starting a new food it can take 3 or 4 weeks to get them on track with it.
Always use a stainless or ceramic bowl for food and water. I wash them every day as if any bacteria sets up on them it can cause loose stools. Plastic bowls can harbor bacteria even after washing so I never use them.