It's a big laydown but it can be made, I think, especially as your intial read was middle pair. I've laid down aces full to quads once (the opponent was kind enough to show me) but that was in a cash game and the betting pattern was a lot easier to read. (I bet increasingly heavily through the streets and he went ott all in on the river). Here it's very difficult because he could easily have 99 or similar and know that you've missed. But...if this were a bluff or semi-bluff he'd have to be a fish because he's got to think you'll call.
The hardest thing about laying down is not knowing for sure whether you've made the right move.