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.
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.