A You're right about the rule. "If you were him" is not correct grammatically. However, the rule is followed so rarely these days that "if you were he" sounds strange. "If you were him" is acceptable in informal conversation. If you want to be grammatically correct and avoid sounding stiff, you can use a noun instead of a pronoun in that sentence: "Heck, if you were Jack, you'd be bummed too, wouldn't you?"