Go to the pet store and get some KMR liquid. Also get a nursing bottle (they should be near the KMR). That poor baby needs nourishment NOW! Heat a sewing needle and put hole in the nipple so the milk will flow. Test it to see if it does. Babies that young need to be fed every 2 hours. They also need to be stimulated to go pee and poo (that's what the Mama does when she grooms them). If you have a heating pad, put it in a box with the baby, set on low. Babies also need warmth, and since Mama is pushing it aside, a heating pad will do. It could be that there is something wrong with the kitten that maybe isn't quite visible, but Mama knows. I'd definitely call the vet. He'd be the best one to give advice on such a small baby. Good luck.