It adds up pretty quickly. For 2008 and 2012 combined, he only got one state wrong. Say there are 10 swing states close to 50/50 chances, and all others are fixed. The chance of getting them all right, twice, are 0.5^(2*10) = 0.000000953674316.

(that's actually not the best method to evaluate him, because he provides estimates of chances and could be better evaluated with something like the https://en.wikipedia.org/wiki/Brier_score that checks for calibration as well, but it's more intuitive).

There aren't ten swing states. It's probably closer to half that. And they aren't 50/50 either. I got 49 right the last election just by looking at the public polls in the days leading up to the vote.