Used negatively after a pronoun presents a problem that has confounded careful users for generations.  Do you say, “Everyone but him had arrived” or “Everyone but he had arrived”?  The authorities themselves are divided.

Some regard but as a preposition and put the pronoun in the accusative—i.e., me, her, him, or them.  So just as we say, “Give it to her” or “between you and me,” we should say, “Everyone but him had arrived.”

Others argue that but is a conjunction and that the pronoun should be nominative (I, she, he, or they), as if the sentence was saying, “Everyone had arrived, but he had not.”

The answer perhaps is to regard but sometimes as a conjunction and sometimes as a preposition.  Two rough rules should help.

1.    If the pronoun appears at the end of the sentence, you can always use the accusative and be on the firm ground.  Thus, “Nobody knew but her”; “Everyone had eaten but him.”

2.   When the pronoun appears earlier in the sentence, it is almost always better to put it in the nominative, as in “No one　 but he had seen it.”  The one　 exception is when the pronoun is influenced by a preceding preposition, but such constructions are relatively rare and often clumsy.  Two examples might be “Between no one　 but them was there any bitterness” and “To everyone but him life was a mystery.”

SOURCE:

Bill Bryson
Bryson’s Dictionary for Writers and Editors
Broadway Books 2008