If the guy leaves the phone on, it can easily be tracked.
The police should contact the cell phone provider's CALEA (communications assistance for law enforcement act) group. They will need a warrant. The cell phone provider can triangulate the cell phone location, often to within a few feet, if the phone is turned on.
If the phone is not on, they can still tell which tower was used for any calls that were made while the phone was on, which may help them to locate the suspect, especially in thinly populated areas.
If the guy still has the phone, you've got him.