I would tell him go to the CEO and say:

"The letter was a forgery, someone in your company is unethical, and you should find out who.

As far as I am concerned I am thinking of suing, but if you simply paid out my leave I would be satisfied, and finding the unethical employee will be on you."

And that's it. If the CEO is as honest as the question makes him out to be, this should be enough, and it's much simpler. If not, then you lost nothing, and can sue, get a lawyer, etc, as all the answers suggest.