The person to track is the mother, and find whether she had a previous marriage. The 1900 Census indicates she was single, but I would still try and establish that as fact.
What does her marriage record to August Valter indicate?
If she had a 1st marriage, then you might assume that partner was the biological father of Herman. If she wasn't married at the time of his birth, then finding who the biological father was might not be possible.
Other documents you could look for is his baptismal record, that may or may not have useful data.