Will has a good sense for numbers, as Grandma Susan says - he thinks about math a lot. But even up until about a month ago, he struggled with the numbers 1 - 3, and any other numbers might as well have been a zillion. In fact, he still gets those three confused a bit - when it's about time to get out of the bath but he wants to stay in, I'll tell him he has three minutes, and he'll argue back: "no, TWO!".
When we went to visit Cate last month, all of a sudden he was counting much higher. I don't know what happened, but I heard him count to six at the beginning of a race he was running against himself, when as far as I had heard before he could only reliably count to two. Turns out he could count even higher than that, just can't pronounce "seven".
Since this video, he can kind of say "seven" but usually just pauses and waits for Erin or I to fill in for him. But he also tries to count even higher than ten - usually goes a bit like this though: ...eightnineTEN!!! sicteen, farrteen, TEN!!