Pituophis are notorious for regurgitating and vomiting. If the mouse is partially digested, then it was vomited, meaning it was brought up after it passed through the stomach. If this is a hatchling, then it is doubtful that it has such a parasite load as to cause this (but it is possible). Some possibilities are:
1. Cage temperature too high or too low.
2. If it has never kept a meal down in it's life, then there is a possibility that there is some defect in it's digestive tract. I had a hatchling Black Pine Snake with such a defect that was never able to keep a meal down and finally had to be euthanized.
3. Some intestinal infection, such as Giardia, Entamoeba, etc.
I would suggest consulting a veterinarian for a proper diagnosis.