How can I track my son's stolen iPod if it was not configured with the iCloud service?

My son's iPod was stolen from him at middle school (go figure, that's never happened I'm sure) while in the bathroom. He does not recall having registered it with the iCloud device, and the serial number from the original receipt from WalMart comes up as 'invalid' for some reason despite this being the actual product serial ID that matches the correct Apple format for serial numbers. We are trying to track it as it's a 4G and has the GPS and wifi enabled.