Unlike the rest of his fellow soldiers, (Oregon National Guard Private Duncan Schneider) has a slight disadvantage because his company's first sergeant is whom?Peter clearly gets the basics of how to use "whom." But like most people, he's thrown off when a predicate nominative comes into the mix.
The predicate nominative is the grammatical reason people say "This is she" instead of "This is her."
The predicate nominative is:
noun or pronoun + to be + noun or pronoun (that's the same person or thing as the first noun or pronoun)
The rule is that the second noun is in the nominative (subject) not objective case. In other words, "she" instead of "her" in "This is she."
So, paring down Peter's sentence, we find the predicate nominative:
The first sergeant + is + ______.
That's why there's no reason to use the object "whom." Peter should have used the subject case, "who," saying, "His company's first sergeant is who?"
What's the moral of this story? NOBODY'S grammar is 100% bullet-proof. So don't let that insecure, "Oh, no. I should know it all and I don't" voice make you feel overwhelmed.