You have given it a good start. Keeping it warm, and allowing it the opportunity to drink with help it alot. Provide a hide boas for security, also make sure it has access to a hot spot and a cold spot. After a day or two offer it some food. A new born rat pink should work nicely. If it doesn't for it, heat it slightly and leave it beside the enterance to the hide. If if feeds, start it on a cycle of feeding every 7 to 10 days. It should come around quickly if the lethargy is due to incorrect care by the previous owner.
Try this. If it does work, we can then expore alternatives.
Warren
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Dr Warren Booth
North Carolina State University
Department of Entomology
3309 Gardner Hall
Raleigh, NC 27695-7613