I wish I were as sure about any single thing as he is about every single thing.
"Single" would fit with emphasis on "any" and "every".
We can't help you with iq tests here.
I'd say that is the correct response,
and avoids using "single" twice.
I'd go further and say that that is how most native speakers of English would fill in the blank.
When I was teaching we had a worksheet for the kids to fill out with suitable adjectives. I was the one to point out that '*******' could easily take the place of all the adjectives. It's a useful back up word if you're feeling a bit tongue-tied, meeting someone important like the Queen or Doughie Squires.