Try putting the puppy on a towel soaked in cold water from the tap. If that makes it stop, you know the problem is heat and can address that.
We did this with our now 4 week old litter of two and it worked like a charm. We found that one of the two was more heat sensitive (and vocal about it) than the other.
I just had this happen with my last litter. They were about 4-5 days early. The pup was crying, crying and wouldn't stop. I took him to the Vet and he had phenomena and sepsis from the umbilical cord. The Vet gave him a shot and shots for all of the pups for a week and he healed up fine.
Listen to his lungs and see if he has any swelling anywhere.
Thanks everyone. It was the heat. They loved the cold ice pack under a towel. The next day it was only 72 F here and pups were very quiet. I was going to try the cold wet towel as I didn't want Mom to eat the ice pack but it's not so hot for now.
