I would see if he will eat a teaspoon full and then let him eat some dry kibble afterwords. Tomorrow if his stool is still loose in the am do the same thing again. You just need to give his tummy time to adjust to the food change, just make sure if he has diarrhea really bad you give him nutrical, karo syrup or some honey and plenty of water to keep him hydrated and not have low blood sugar issues.
The vet checked him for parasites right???
Here's a helpfule post
http://www.yorkietalk.com/forums/sho...canned+pumpkin