SS killed allot of people for finding partisans.Entire villages was burned sometimes.With this things cannot be attain sympathy from the population.Also when such "events" happen frequently,very few strangers get help,and german spies was very difficult task for getting trust/support from local population in attempt to find any valuable informations.
Maybe its possible for him to infiltrate the buildings of the allies to delay unit creation and stuff like that. Slower upgrades, global cooldowns take longer if you infiltrate into a HQ etc. He would have a 50 %(or lower/higher) succesion rate so not all attempts would succeed. I don't know if this is possible though.