Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that one of your puppies seems to have an upset stomach.
Is he still eating well?
Is he crying more or does he still seem content?
Any change in diet for the bitch?
Was mom well vaccinated before she was bred?
Was mom on a good worming schedule before being bred?
Have the pups been wormed yet?
If the pups are still eating well and seem otherwise content this may be something as simple as a change in diet affecting the bitch's milk and upsetting his stomach, or gastrointestinal parasites (which are very common in puppies).
Make sure mom is getting a consistent diet, no rich or fatty treats.
And if the pups have not been wormed yet they should be wormed with pyrantel, a gentle, broad spectrum wormer safe for young puppies. They should be wormed every 2 weeks until 12 weeks of age and then monthly.
More worrisome at this age are secondary bacterial infections or a viral infection. Watch the pups very closely for loss of appetite, vomiting, a nasal or eye discharge, or bloody diarrhea.
Make sure the pups aren't getting chilled or too hot.
Makes sure to clean the puppy area thoroughly, and clean up after the pups.
Try to limit visitors and if anyone is handling the pups they must wash their hands thoroughly and never wear any shoes that have been outdoors in the puppy area.
Please let me know if you have any further questions.