Awwww! Poor Puppy. First thing I would do is call his Breeder. Did he come with a few days supply of his usual food and instructions of feeding? Sometimes just the stress of travel can play a part. As well as different water.
#1 - Call the Breeder. They will most likely be there to help with any and all questions and problems.
#2 - Holly! Are you there? (She's the Guru of feeding guidlines.)
#3 - What type of yogurt are you giving him. I use Dannon plain & all natual.
#4 - Potato - boiled with skin (I use organic red) and smushed & added to food will help the runs & tummy in general.
#5 - Lots & Lots of HUGS and a GREAT BIG WELCOME to BPO.
#6 - Pictures are a MUST!