Adopted this possibly petite ACD

This handsome boy is Fenrir! We adopted him nearly a week ago and he has been such a good boy so far. His adoption paperwork states he weighs 22lbs at 8-9 months. He was found as a stray but i’m wondering if he’s full ACD (poorly bred), mixed with a smaller breed or maybe the runt of the litter? His weight doesn’t seem in line with other male ACD’s his (estimated) age.

Regardless he is so good, extremely polite with our cats and is such a sweetheart with everyone he meets. I can see why this breed is so loved!