I'll say I have always been very firm with him when he has needed it. But, he doesn't blink.
My husband on the other hand finds it hard to reprimand. But, does try because he knows this is serious.
From the AKC site: "The German Shepherd Dog does not give affection lightly and is known for his dignity and stature; it is also known as a "one-man" breed for its tendency to display serious loyalty and fidelity, especially to its owner or main caretaker."
Have you tried staring him down?