Can I feed baby calf milk to newborn rats!?
Not knowing the age, I can't give better advice, but it's best not to use milk at all. If you have nothing else, you have no choice.
The First Dragon
If the rat has teeth and its eyes are open, you can transition to solid food. People drink cow's milk, so I guess rats can too; but it is not considered best for very young babies. You are right to massage the anus; most very young mammals require this. I would be very concerned that the rat hasn't pooped or peed in two days; but I wonder whether it is possible that it peed and you didn't notice it. It would be very much pee! You could try feeding it more water with a nipple or dropper; make it warm water. Even prune juice should be okay, as rats do eat fruit. Don't be alarmed if it has diarrhea after the prune juice. It's still better than being constipated. You also might massage the belly.