First I think you are doing a wonderful job with Nebo.
Keep it up, and see what happens.
(Maybe he is having a bad day.)

My Aunt/Uncle owned a GSD that disliked men.
The only 3 men she would accept was
my Uncle, (who she adored), my dad and my grandfather.
She also was the best dog/and most obedient trained
dog they have ever owned. For some reason her
protective instincts kicked in when it came to men?