You need to take this kitten to a vet immediately. Really, right now. Find an emergency vet, or call the vet back and tell them what is happening and that you need to be squeezed in today. A kitten in this kind of condition has something seriously wrong, and kittens can go downhill very fast and not recover. If the kitten doesn't see a vet for the next 3 days he may be dead by then.