I wouldn't hold my breath for his extension. It seems incredibly unlikely at this point - and after doing the show for so long, his voice needs a break.
I wouldn't be surprised if he came back for a limited engagement in a year or so, but I think it's safe to assume that he will not be extending.
Doesn't it seem likely that his current standby or understudy will just take over?
"If you are going to do something, do it well. And leave something witchy."
-Charlie Manson