Hello Rudolf,
If you have a document stating that he was your real father (that is how I understand what you wrote), I would write to the City of Northeim, stating what kind of information you are looking for and put a copy of that document with this letter. Or is it not even confirmed that he was your father?
You will have to write a letter, as inquries per E-Mail are sometimes not accepted. Most important will be:
- state clearly what information you are looking for
- enclose helpful information you might have, like known adresses and reliable dates you have.
The information you are looking for will most probably not be found in the Internet.
Another idea might be to ask at the German embassy or a consulate about it, perhaps they might be able to help you. At least you might be able to explain somebody your request in your native language, to avoid misunderstandings. There might be other norwegian-german institutions that might be able to help you, as well.
Stefan