I am not sure, however:
His collar stripes indicate, of course, that he is an officer
As for the officer rank. It doesn't help much that the picture is black and white. Also, I can't really make out the shapes of the objcets on the shoulder straps. My best guess is this:
-2nd Lieutenants had no extra shapes on the should straps -1st Lieutenants had one shape. -Captains had two shapes. -Majors had a very fancy fluffy spiral kind of shape so that is out of the question.
Since I see two shapes on the shoulder straps, I am assuming he was a Captain (german: Hauptmann)
Edit: However, if the middle shape is a number, then we would have a number and a shape (and by shape I always mean like an object like a pin) then it might be Sergeant (Feldwebel)
Couldn't be of much help I am afraid