Sounds gifted to me....but I'm biased as my ds sounds similar to yours.
Even though my ds was an early reader, it really was his early math that caught my attention . Maybe it's because I'm not mathy in the same way. I remember my ds doing the same mental math figuring when he was about 3.5 as your ds (his first epiphany: that there were many ways to get to 4…2+2, 1+1+1+1, 2+1+1 and so on).
I love to hear my ds5 reason out equations...and he loves to be challenged with new ones. He likes things like "find a way to get to 120 adding 4 numbers including 18" (random example). He doesn't know most math facts by memory, so he'll calculate according to the ones he knows.
My ds also asks questions until I run out of answers or make something up
. We have spent hours of car time on math talk..sounds like you will, too.
