I would pull the babies off of mom - it sounds as tho her milk may be going bad and this little guy is having more problems than her siblings with digesting it. If you can get them inside and start them on Kitten Chow mixed with Kitten Milk Replacer (not cow's milk), I think this will turn around with no further effort on your part. You may want to withhold food overnite to give her tummy a chance to settle. Also, mix a spoonful of plain unflavored yogurt in with her food to help restore the natural flora in the gut. Offer plenty of fresh water and let me know if she is still having the problem by tomorrow nite.
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