Firstly keep the pup warm and try syringing special puppy milk into its mouth. It isnt suckling so you will have to syringe a bit at a time. Try latching it onto the mother first though just incase. You will have to seperate it from the litter so it doesnt get pushed out by the others. Keep it in a box somewhere quiet on a hot water bottle covered in towels and feed it every couple of hours.
I have had this happen a few times and found this helped alot.
I hope your pup pulls through.
Best of luck.

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