He simply needs to be a mod. Being a mod would allow him to move the odds dynamically as a pile-on appears, just as real-world bookies do. Anyone who had already bet would keep the odds they'd laid their bet at but he could change the odds for bets yet to come.
Another aspect to being a mod is that he would be able to set a limit to control how much he's prepared to lose. It would effectively give him the power to say "no more bets". The software is already in place.