Sorry to hear about your pup its rare they make it pass 4 days or so as they just can't draw the milk as the other pups. You can hand feed but you should also see a vet, get his advice and let nature do the rest.
Personally I wouldn't do it as I would hate to see it all go wrong later. Hare lips (I've heard) can survive but not cleft pallets which can develop later. They say it's breeding but I believe its carried in certain lines that just doesn't click.
What breed is it?