This. Be honest with him. Tell him you've been playing with small change until now, and have not had to worry about the impact your trades would have on the market, because they were never that big. I'd sit him down somewhere and make this a professional conversation...a talk you'd be giving him, advice, if you will, which would clarify things.
My advice is to tell him that you'd be willing to do with this with 20btc to start with in order to get a feel for the new level at which you'd be playing the game. Have him understand the risks, have him accept them, too (these are two different things).
Tell him that whether he should be given more than 20btc to play with, and whether he should be set up with an office, should be determined after a few months. There's going to be a learning curve for you, and you should make it clear that on principle, you want to be judged on the merits. You don't want to screw him over, even unintentionally, and you also don't want to kill yourself with stress over this.
He'll appreciate your candor, and even if you fuck up, you might get hired by him for another job. Apart from that, you'd be doing him, since he's blinded by greed at the moment (and thinks he'll make half a million in a month), and you'd be doing yourself a favor, too, because you wouldn't be trading with a lot of anxiety and most likely making stupid mistakes--and basically go gray and bald like the presidents within a few months.
Don't fuck up this once in a lifetime opportunity. Best case scenario is you double his money in a few months, and he tells his friends, and they start getting in on this. Even in a worst case scenario, so long as you maintain your integrity and candor (or at least the appearance of it), and so long as you are extremely organized with updates, paperwork, and so on, you'll make it even if you don't do turn a profit with his money. People with that much money know the risks involved in these things.
