Congrats! 
How many runs was that? (I’m still looking for him, now and then - and I’ve done 265 runs over a few years.)
Here: Rainbow Portrait: The Hunt for Sir William (These are the facts!)
It says: “Only TWO unique spawns can spawn in one game in Whimsyshire and Evil Oliver and Super Awesome Sparkle Cake are always one of them. This means if you meet ‘Tubbers’, ‘Midnight Sparkle’ or another random unique spawn before you run into Evil Oliver or Super Awesome Sparkle Cake, then you know it’s guaranteed that there is no longer a chance that Sir William can spawn in the game and you should therefore just leave- and remake the game and start all over with a new Whimsy run. Knowing this definitely saved me some time on non-necesserary clears!”