you didn't mention what species it was...it it an eastern milk?
where was it found?
A milk snake at 1.5' and big around as a pencil can easily a small pinkie mouse. Spilt the nose of a forzen thawed pinkie (to get a little brain matter on the nose) and place the milk snake in a very small conatainer like a deli cup or even a ventilated vitamin conainer, with the pinkie and leave overnight.
That's the first thing to try. It very well may work. Many young milks can't resist the strong odor in a small area like that and the darkness gives them the security they need.
If that does not work...next step is a small live pinkie...
good luck