Question for you ... if you're leaning towards adding pumpkin to his food, as well as probiotics and other digestive aids ... would you consider switching to a raw diet? It's not as hard as it sounds, and you'll have a much healthier dog in the long run.
Carmspack, Gatorbytes and Lauri & the Gang have LOADS of info to help you out on this. You should check out some of the threads in the forum on raw diet.
Good luck with your puppy, runny poops are not fun to pick up!
Marionís Zoo-Kyleigh, Raylan-cat, Echo-TAG,