| what food is he on now? then second, I wouldn't feed a puppy food, per se. Try one of the all life stages foods.
and I'll one to CMax's list: a bloom of the naturally occurring bad gut bacteria that overwhelmes the good bacteria and throws the system into upset. The colon can be a dogs stress organ (lots of nerves in there) so what we might not think as stressful could be stressing out the puppy resulting in the out of whack bacteria and loose stools.
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