I brought my pup home about a month ago. He did the same thing. He was 9 weeks old.
He hardly pooped at all and when he did, it was runny. It worked itself out with Yogi. I used the same food that he was used to but the vet said that he was probably just upset and stressed and that he should be fine once he gets adjusted. And he was. Now, he's a pooping machine
I wouldn't worry. When you take him in for is check-up, just mention it to the vet.