You can also check with the Emigration ("Auswanderer") museum in Bremerhaven . There is the chance that there will be a record of when he left Germany, which could provide you with some additional clues. I think it is searchable online but have not looked at it for quite some time, so am not sure, but the place is intended for people to be able to find information. I found one of mine listed there, although the other 6 were not, but tat one was a valuable clue.
Not all German archival authorities are friendly or helpful. The one I contacted tried to tell me my theory could not possibly be correct as to my connection with a particular person. But I persevered. She was wrong, and I was right, as it turned out. You have to persevere, as Florence has said.