I am really sorry to hear about your situation and I can understand that you are very worried.
As far as I know, it happens that the border police is taking the phones of people who "irregularly" (try to) cross the border to Germany in order to get information about a person's background. They may also detain a person for a few days/weeks. I am thinking about the best way to find out about his location. One option might be to contact the police directly. Maybe the Tracing Service by the German Red Cross can also help if you don't hear from him very soon - what do you think @mbeon-Éanna?
Stay strong and please keep us updated,