Are you thinking more along the lines of humiliation? Not that I think that's the best idea, since it won't change his behavior.
Let's see, you could make him shave his head if he's vain about his hair. You could make him go out to dinner with you; dressed like a girl. You could make him eat a lot of something he doesn't like.
The only problem with going this route, is that you'll have to do something humiliating if you don't lose the weight.
Personally I'd make him take out a full page ad in the local newspaper admitting what a jerk he was.