Oh man. And he says this right before your target date? Ai.
If he's not down with it, though, there's probably hell to pay, so I guess you just let him run his own numbers and make his own projections... and then sit down to have the talk you probably needed to have a year ago.
Really, even if your spending changed this year, you could just change it back, right? You already *know* you can live on that amount, you've done it. So this balking suggests to me that maybe he's never been convinced about this early retirement thing, and just kind of hoped it would go away. Now that it's actually close, though, and he knows that you really mean it, he's pushing back.
Or maybe his identity is really wrapped up in working, and the thought of not having that, -- or of not having "a place to go in the morning" -- is suddenly giving him the willies?
I suspect this isn't really a numbers problem, though you may work it through by talking numbers -- I think you need to interview him pretty closely about what stopping working will mean to him. I mean, ask several times in several different ways until you're sure you've got it all out of him :-)