Such a young little pup. I would start thinking about a new food for the future: example Natura, Canidae etc. Although with the digestive upset I would do this very very slowly.
Right now, I would take the pup off the current food you are feeding, let the belly rest for 1/2 day (you don't want to starve a puppy - and never withhold water)..then start back on a homemade bland diet such as white rice boiled with either boiled white chicken meat or boiled lean ground beef. Give some yogurt in between meals (I use only natural organic yogurt from liberty found in the holistic section of your grocery store)...in the future servings of yogurt you could add some ground organic flax seed. Make sure you feed small meals throughout the day.
What I have used in the past on my adult dogs (I havent had a puppy for quite some time) is Slippery Elm bark which soothes the digestive system...and then dandelion root to support the liver and aid in digestion. You can find these supplements in any store that sells vitamins and herbal supplements. I have had great success with Slippery Elm Bark...but, first do a google search and make sure it's something you can give to a puppy.
For diarrhea since you know it's not worm related could be caused by emotional stress - look for any triggers in the environment...maybe just too much going on at one time for the new pup.
Be very careful if giving Pepto-Bismol, which is a preparation of bismuth subsalicylate. It is effective but contains synthetic red dyes and saccharin, and salicylates can cause allergic reactions in sensitive individuals.
A good natural remedy for diarrhea is carob powder, available at health-food stores. It should give you guidelines on the package of how much to give for weight, mixed with some applesauce and honey to make it palatable. Take it on an empty stomach with acidophilus. Carob is very soothing to irritated intestines. This is something to discuss with a holistic Vet though.
Food allergies I would talk to a holistic vet about this possibility.
Bacterial overgrowth in the small intestines - by giving organic plain yogurt you should be able to get more "good" bacteria in the gut.
Make sure your not over feeding your puppy. This is common with puppies, get too much to eat and their little system can't handle it. I never free feed any dogs/puppies. They always have scheduled feeding times so I can control how much they are eating - puppies I normally have fed 4 times daily small meals. On most commerical puppy foods they give feeding schedule per age and size.
I suggest your finding a good holistic Vet in your area. I can't begin to tell you the people our holistic vet has helped where conventional vet care didn't do anything for the poor dogs.
Always talk to a Vet before giving any supplements. The canned pumpkin, yogurt and bland diet should be OK...but, do research on the other herbs first...I know their safe with adults...but, I havent puppy tested them yet.
