Question: Can you or your father by any chance provide any information on this painting's provenance? That would help a lot if it could be related to a particular sitter or found to have been exhibited under a certain title. It may also be helpful to look for catalogs of student exhibitions held by the Munich Academy when he was a student there and see if he exhibited anything in one of those shows.

EDIT: Also, it seems at least one catalog of his work has been published when he had an exhibition in Berlin:

Schellemann, Carlo. Carlo Schellemann, ein Maler und Graphiker aus Eggenfelden/BRD. Nationalgalerie im Alten Museum. Berlin: Staatliche Museen zu Berlin, Verband bildender Künstler, 1984.

So this may have some info on this painting and even if it does not it may have other useful references on the artist, e.g. information on previous displays of his work or what galleries he exhibited with.

EDIT #2: Something else I might as well mention: depending on what your father has the time and energy for, he could also contact the Wende Museum-they're a museum in California specially devoted to Cold War material culture with a focus on East Germany so they would likely be able to help him better figure out how to research this painting.