If this pattern has only been gone on a few months, then I'd still recommend the honesty approach. Be firm in saying what you know to be true - what your boss is asking for can't be done. It's possible that being really direct and firm will cause your boss to rethink. If not, then he still needs to hear how seriously you take it. I don't know what your situation is with respect to other people in the company but if there is a third party that you and your boss both respect, perhaps ask for some sort of mediation - a sit down with the three of you.