AirTag sharing work around

One other issue of concern is Airtags are designed to alert people if they have an Airtag signed into a different account that's with them. It's a privacy feature. I invite you to read the details but I wonder if this might present problems. What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you - Apple Support

can I track his AirTag in his suitcase

If you have a device signed into his iCloud account then yes. Otherwise, no.

If you want to track his luggage with your device you will need an AirTag attached to your iCloud account.

AirTags can be found in Find My only on a device signed into the same iCloud account as the AirTag.