i have a cat who won't use a box that is totally clean. took him to the vet once and he refused to use the litter box they had or to go anyplace else and held it in for 36 hours! poor guy! i guess it is good that he doesn't go other places but i have needed to collect a clean specimen of his pee for testing a few times and it was tricky.

anyhow hope your shadow turns out to just be skinny from being outside so long. worms can cause that and is easy to treat and that he gets used to using the box. such a pretty cat.