I dont have a cat, but i looked online and it looks like it might need a vet check up, could be a sign of a UTI.
Other things to think about.. has the box been moved? Does he poo like right next to it, or is it off somewhere? could he just be missing the box? does he always poo in the same place? you could try and get another box and put it next to the first one

Does the litter box have a cover? my aunt had a cat that wouldnt poo in there covered box, i think he got scared cuz he couldnt see what was going on outside.
Do you have more than one cat? Sometimes cats will suddenly decide they dont want to share a box anymore.