When changing puppy food?

It doesn't sound like a food problem to me. I have a hunch there is something else going on here rather than a dislike of what you are feeding him. It is not normal for a dog that has previously been a good eater to all of a sudden decide they don't like what they have been eating, and start having diarrhea. When switching to a new food, you need to do it over at least a 5 day period, and this would apply to a food within the same brand also, as the ingredients are different. If this was my dog, I would definitely take a fecal sample in to the vet for analysis.