Proper means grammatical. "To be" has a subject and a predicate nominative after it, not a direct object, so "he" is correct there, not "him." Now in informal English, one often hears "It was him". I am not objecting to "him," informal English is fine by me, but to reject the "proper" English "he" in favor of "him" is unfair. Both should be accepted.
I think Stuart gave an excellent explanation for why "him" is not proper.
He didn't. His argument is basically "I've learned this minimalistic set of rules in school that make sense to me and now I'm trying to enforce reality to conform to them even if people obviously use other, more complex, rules."
It's very common among "grammar fans" or "language mavens" or whatever you want to call them; and it's ❤❤❤❤❤❤❤❤. For example, if pressed, he wouldn't be able to produce a solid definition of "proper" - English doesn't even offer the comfort of a prescriptive authority like L'académie française.
Imagine seeing this in scientific fields other than linguistics: people yelling at quantum phenomena, calling them "improper" because they don't conform to the laws of classical physics they've learned in 6th grade.
Precisely. To be correct, it must read, "It is not he, it is someone else."
The test for this, he vs. him is to continue the sentence: "It is not he.... (...who open the door,) (...who bakes the bread,) (...who is the father,) (...who helps the old lady,) ... it is someone else"
This is because subject pronouns are used after "to be" verbs when they rename the subject.
Informally one can say, "It is not him, is is someone else.", but technically this is not correct.