I'm willing to bet it was definitely fear. Just don't give up doing this>>"I told him to leave it and tried to shift his focus to me, but it was a struggle."
Nothing says he was trying to protect you, everything was fear based. Just keep up the obedience and exposure. He just showed you his nerve threshold at this age and point in time/situation. Now you know.
"An excess of size is an unserviceable feature for breeding, his powers of endurance, his speed and the smartness of his movements suffer in all circumstances. Giants are never nimble."-Creator of GSD