switch slowly. Abrupt change will cause the runs too.I don't think its a parasite or illness. I have two pups. About 16 weeks. The breeder of one of them recommended Purina Pro Plan Puppy. I thought I'd try something where corn was not the #1 ingredient. I went with a more expensive brand that I prolly won't name. Lamb and rice. The stool goes gray and loose right away. Didn't suprise me because changing foods and all... But now it's been two weeks and still loose. I think I will go back to the pro plan. They were doing fine.
X2 for the Fromms Large Breed PuppyIt does sound like something other than the food switch, unless you switched too fast and got the gut out of order. Try a canine probiotic. Check for coccidiosis and goarfish.
Along the lines of food, Fromm Puppy Gold is great. Can't buy it just anywhere. Great customer service and they have a breeder program.