Puppies are like people and some times poopy happens stress, different diet can all trigger it. Puppies are also very resilient so dont worry and give it a few days things like this do pass. puns intended. By all means get ehr to the vet but dont worry.
I got Kona from a dairy farm at 7 weeks (I know, too young..) where he was being fed really bad food. When I first got him home we switched him to his new food, which did not go well. I resorted to feeding him rice, pumpkin and a small amount of cooked eggs as a substitute for his old food (They didn't give us any of what they were using) Over about a week or two I switched him on to his new puppy food, mixing it in with the rice and eggs. I also cut back on the treats I was using as they were very strong (dehydrated meats) and were upsetting his stomach even more.
Good luck and congratulations on the new puppy!
How great he can make a new start with you, all though the situation is not so nice now.
I should take him to a vet on first place. Don't take risks by trying yourself, a good start is important. He had a bad start, but now you can do better.
What about food, I know that grains are not good dog food, and most cheap foods are made with a lot of preservatives and grains, to fill it up... grains are also heavy on the stomach.
Also the mating with his age, I think much to young.My Bo is now 1,5, but actually still a puppy in her head, and also her body is not full mature. So I agree what Trifan say's. Here in the netherlands they say even 3, that they are fully mature.
Good luck with your new Puppy, I hope he will be ok soon, and that we can hear nice story's about him!
There is absolutely NO REASON to have to breed him by 1.5 or 2 years old. There is no age on first breeding, except that it is really irresponsible to breed anything that has not had certain health question answered and one of those is hips, which can not be accurately rated until the dog is 2 years old.
As far as best food, there is no such thing as the best food, except what works best for your individual dog. What is best for one dog is not necessarily best for another. If the loose stools continue, you may need to switch to a food that works better for your dog. And I agree about Blue Buffalo, but there are hundreds of good foods that are not Blue Buffalo.
Gotta love 'em.