I've never done a Kong so I don't know. I feed my 5 month old lab 3 cups a day, 1 1/2 in the AM and 1 1/2 in the PM. This is the same schedule she was on when I got her from the breeder at 10 weeks. Same amount of food too. She is on the chubby side at 50# already too.
Couple of things, this is the age of teething which can cause runny stools. My other guess is you are feeding too much. Cut the pup down to 2 meals, 2 cups per meal. And Blue Buffalo isn't the best food but if it works after cutting the amount down, I wouldn't change. I'd probably only do a PB kong 1x a day. Or just use some of the dogs kibble from it's 4 cups with a little canned instead of PB.