I'll just note that if he is living in Germany, his account should be set to Germany. In that case you cannot be part of the same family group since all members have to have their accounts set to the same country. Change Purchase Sharing settings in Family Sharing on Mac - Apple Support
The sections below are copied from the USA Apple Media Services Terms and Conditions document but I suspect they are very similar for all versions of this document --> Legal - Apple Media Services - Apple - Last Updated: September 16, 2024
From Section A: "Our Services are available for your use in your country or territory of residence (“Home Country”). By creating an account for use of the Services in a particular country or territory you are specifying it as your Home Country."
One possible reason for a subscription not working is:
From section J: "Terms found in this Agreement that relate to Services, Content types, features or functionality not available in your Home Country are not applicable to you unless and until they become available to you. To see the Content types available to you in your Home Country, go to the Services or visit https://support.apple.com/HT204411 . Certain Services and Content available to you in your Home Country may not be available to you when traveling outside of your Home Country."
If he just joined your group, have him restart his device so it takes a fresh look at configuration.
Not all apps can be shared. Some can be shared but may require a family level subscription.