You are right to be concerned about this puppy being taken away from mum too early and it will need special help not just in supplementary feeding but also with likely future behaviour.
Firstly it is really important that you take pup straight to your veterinary surgeon for a full health check and advice on the correct way to wean and supplement the feed. They will be able to supply you with specially designed puppy milk as cow’s milk is not suitable.
Once puppy is healthy and strong you will need advice from a good behaviourist on how to make sure puppy knows when to stop biting. Mum dog allows very young puppies to do all sorts but as they get older she begins to put them in their place and they learn what is acceptable dog behaviour and what is not. It is important that learn how to do this especially with a breed such as a pit bull. This needs to begin around six weeks of age but you will need professional help with this as it is not just a case of shouting at it. Ask you vet to recommend someone who can guide you.
Good luck
Rosie Barclay