He's probably already in shock from being taken from his mother, and odds are he will die IMO. You can try using Esbilac formula puppy milk (it's the kind usually recommended for wild animals). It keeps forever pretty much, so you'll have it in the future if something like this happens again.
I'm drawing on possum experience here, but I imagine a rat would be more suited to that formula than a marsupial.
EDIT: You can usually find that formula at Petco/Petsmart.
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