Possible causes
Stress new environment
Change of food from what he was used to
Food too rich Ie adult food
If there is no vomiting and the pup is happy and content i would think it would settle on its own, make sure the poo has no blood and dog doesnt develop vomiting.
They have very sensitive stomachs so any food change will disrupt them. You might need to fast the pup for twelve hours to give the gut time to rest then start back on small, bland foods for a few days then slowly introduce puppy food, Dont give them milk either cause they can be lactose intolorant