[moondog]

How long has this been going on (days, weeks, or months)?

I would take this very seriously. Diarrhea is not normal, and it is not a behavior problem. It is not because he is getting too much water, it is because his bowel is not functioning properly to absorb the water he is getting, which can lead to dehydration. If multiple stool samples have been checked and nothing was found, then you and a good vet need to take the next step in determining a cause. I would request a complete blood panel be done to eliminate some more serious causes and ask the vet to check for the possibility of a partial obstruction if he hasn't already.

Getting to the bottom of this is important. You have a young, developing puppy and it is important that he be able to utilize the nutrients he is receiving. Switching foods willy nilly in hopes one will work can cause an increase in intestinal inflammation and make things worse. If other causes have been eliminated and it is thought this is food related, he should be put on an elimination diet to figure out what foods he does best with.