I think worried is good-not that I want to worry you more

Just I think it's speaks that you are a good owner if you are worried
Hydration is key-you can check by doing a 'pinch test' This link has instructions and info on the test
Symptoms of Dehydration
and symptoms of dehydration.
In the meantime before you get to the vet ensure your pup is hyrdated and belly is full. You can ascultate the stomach by ear for gut sounds-listen to your other dogs first-press your ear to their tummy for a baseline of what a tummy sounds like-then listen to your pup.
If pup refuses to eat you can mash food up small and mix with water and syringe feed it. If it's bad-we can tell you what you need and how to give sub-q fluids.
Seeing the pictures Id' guess the pup is 4 weeks or so. BUT I'm not a vet, just a dog owner

From what your telling me none of the emergency procedures I just typed are applicabe yet and Monday should be cool to get the pup in to the vet
Let us know if things go downhill and we can help; but it sounds like the pup is doing well and you are doing well for the pup