It's too cold for him!
My mother was worried and told me to bring the dog in after he did his business the other day, when it was 32 F. I laughed.
Now it is 4 F, wind chill near -10 F, and he never looked so energetic. Let him out and he gets the zoomies all over the place, stands face first in the wind, still sticks his head out of the car window going over 40 MPH! I am willing to bet that if he said something, it would be along the lines of "don't care that your toes and fingers are numb, just throw the ball again!"
I can't remember the last time I was so active in this kind of weather. Ah, the perks of owning a dog.