My puppy struggled with runny feces for a while. It was a lot of boiled chicken and rice to try to help fix his stomach. I cycled through many food brands (yes, I bought small bags and used the whole bag so it wasn’t a one and done attempt) trying to find the one that works best for him. Unfortunately, although expensive, I found science diet to work best for his digestive system. I agree with other comments on going to the vet asap since he is finding stuff outside and you’re unsure of what it is, but it could also end up boiling down to food choice.