having a valid passport is naturally the first condition for having any visum - for family reunion and for everything else.
The uncle can try to make an application to the embassy for releasing him from the duty to have a passport. The problem can be that his reason not to go to turkish embassy is evidently fear of prosecution which is an asylum reason and can be normally only checked by BAMF in Germany but not by the embassy. And there is no german asylum procedure from abroad.
You can try to contact the Bürgerservice of the german foreign ministry (Auswärtiges Amt):
- Tel: +49 30 1817 2000
Montag bis Freitag (außer an Feiertagen) von 9:00 bis 15:00 Uhr
- Fax: +49 30 1817 51000
- E-Mail: email@example.com
- Postanschrift: Auswärtiges Amt, 11013 Berlin
They also have a contact form on the Website of the ministry.