Well, except he only knows three numbers. And those three numbers aren't related to each other at all. Like his affinity for yellow when it comes to colors, Will has preference for two, six, and nine for numbers. I don't know how he came up with these options (other than two...that one at least makes sense), but it's hilarious to watch him "count". This morning he was counting the blueberries on his tray: "Six!" he would proudly proclaim (nevermind that there were only four). Then he'd add a few more and declare that there were "Nine!" He also likes to count things in his books. He will point at items (or better yet, take my finger and use it to point) and count out loud: "Two. Six. Nine. Six. Two. Nine." until he addresses all of the items. It's too funny.
Will clearly knows that two, six, and nine are numbers because he consistently uses them to count or if you ask him "how many do you have?" he will reply with one of these. However, he just doesn't seem to know that there are MORE numbers available to him.