Ha! What a funny idea for a bet! I guess that's one way to motivate yourself.
So what do you get if you "win" the bet? Hehe. But anyways, you're right, the first week is very drastic--in my first week of weight loss, I think I lost ~10 lbs! But I also started at a much higher weight than you're starting, so I would say that 5 lbs is a reasonable guess. And hey, that would put you in the 200s! Pretty sweet!
Best of luck with the weigh-in! I really hope you've put some kind of wager on this bet, because he is going to LOSE.