how to find out his serial number:
http://support.apple.com/kb/HT1459
From the article:
In iTunes 7.6 or later
1. Hold down the Control key and choose Help > About iTunes (Windows) or iTunes > About iTunes (Mac).
2. Release the Control key.
3. As the iTunes and QuickTime version information scrolls, you will see the last connected iPhone's serial number and IMEI.
It says iPhone here, but it also means iPod. Though it only displays the number of the last connected iPod, so he ever connected is sisters iPod (just an example of course) since his got stolen, it would display her number.
Other ways:
-On your iPod packaging
If you still have the original packaging that your iPod came in, you can find your iPod serial number printed on the barcode label that's affixed to the packaging.
-On your iPod invoice or receipt
Your product invoice or receipt may list your iPod serial number.
Hope this helps, if he still has the packaging, he could even proof it's his.