Yep even and odd numbers and what a cool way to think of it! All on his own! Triple WOW!
I have a great book for him "The Cat in Numberland."
I don't recall when my boys figured out odd or even or if they figured it out on their own or was taught. They both were adding and subtracting negative numbers at 4-5yrs old. The older one figured it by studying a thermometer and noticing numbers less than zero. the younger one....I think he heard his brother and figured it out for himself. I don't know how common it is.
