how do I delete grandsons ipod and add my own?

What do you mean "delete your grandson's iPod"? Delete it from where. Are you suggesting that your grandson's iPod is recognized by your iTunes and yours is not? It would not be necessary to delete your grandson's, because iTunes can recognize different devices. If yours is not being recognized, it could be a problem with driver installation. Has your iTunes ever recognized your iPod? What computer OS are you using? What is the exact version number of iTunes you are using? Have you attempted any troubleshooting? This support document provides information about troubleshooting the iPod not being recognized by iTunes. If iTunes doesn't recognize your iPhone, iPad, or iPod - Apple Support

Where are you seeing your granson's iPod? If it shows under Edit > Preferences > Devices because it was once backed up to your computer then you can remove that backup, but you probably don't need to.

What happens when you try to connect your iPod to iTunes? See Re: iTunes 12.4.2.4 won't show my phone if your device doesn't show up in iTunes.

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Restore to factory settings/new iPod

Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

Your son/his ID will have to be used to turn off FindMyiPhone/iPod in order to activate the device

Turn off Find My iPhone Activation Lock - Apple Support