Mister Monkey Man
Cotton-Headed Ninnymuggins (a term of endearment--not an elfin slight--because he has the fluffiest cottony hair on top of his head... lately, we have shortened it to Ninnymuggins)
Pounder/Paul Poundcake (after Paula Poundstone not because he is witty--yet--but because he pounds a lot)
Snot-Nosed Mouthbreather only when it is literally true (because it's kind of funny to say in a loving way as in my SweetSweet Snot-Nosed Mouthbreather)
See below for Will's reactions when I call him a snot-nosed mouthbreather...
...actually, I think he was just yawning here, which reminds me we also call him Yanni.
Also, I am happy to report that after intensive training, he now also responds to Will and William and WillWill.