Nobody can "force him out." This is a fallacy. He was nominated to be the Democrat Presidential candidate. Period. There is no mechanism for removing him from the ticket. The only way to do that would be to have 50 states re-write their election laws, the democrat party suspend their rules, and hold another convention. It is not going to happen.
I really wish that people would quit dragging out this utterly ridiculous idea of Hillary being the candidate in 2004. She isn't. Kerry is. Period.
Oh, wait, there is another way: If the clintons assasinate Kerry and Edwards, then the dem party would have to put somebody else in the position. That is, of course, equally ridiculous.
I must have missed it, what law is it that says he can not quit?