What's wrong with my 1 week old kitten?

Out of a litter of 6 kittens, he is the only survivor. His mother and father both have had all their shots. One of his sisters was born deformed her skull not fully formed together and one of her arms missing joints (i have pictures) and most of his other siblings had full looking bellies when they died. The mother has been taking good care of them though. After the 5th one died, I've taken this last kitten away. I've fed him KMR formula and tried stimulating his to use the bathroom. He peed a lot, but he wont defecate. He also has the big round belly(not hard at all) and up until now, his mother has been feeding him. Something that may help, is that the father had very strange coloring when he was born, as some of these kittens have. The father LOOKS black and white now, but when he was born he was blackish/dark blue and white, but the blackish color turned lighter as it got lower on his back into this very light gray color(maybe something wrong with his genes?). The older he got, the more it faded. This kitten is exactly the same, but with no white underbelly. His mother was adopted from someone else, and she is a black calico(no white, just orange and black) The birth of these kits was normal, but something strange is that she still looks kinda pregnant. All the mother cats I've ever had always looked thin after giving birth. That's as much as I can think of to write. I know its a lot, but I'm hoping it will help to get an answer. I cant take the kit to the vet for another couple days (waiting for a paycheck) so I want to keep him alive till then.