Is she nursing the puppy?
Momma may be concerned about the baby because he is small. She may want to keep him warm. A cold pup wont nurse.
Take some socks and add raw rice - then warm in the microwave and set them in her nest with the pups so they can cuddle against them if they wish.
Just keep an eye on their interaction to make sure he does not get hurt accidentally.
Please let me know how everyone is doing.
Sincerest best wishes,