If Bill Clinton could be the nominee, so can Kerry. For Kerry to drop out, would be humiliation at this point. It would also admit that he was wrong, although of course he wouldn't say it.
He can announce health problems. Toricelli dropped out based on polls showing he would lose, and he was not humilated by the experience.
For Kerry personally, I agree. He -will be- humiliated. This -will be- personally devastating. He deserves it.