Are you using the same litter he uses at home? And exactly the same food, and did they think to bring any of their water over in a jug?
he may not like where the box is placed, it could be too noisy, or is not clean enough, some cats are very fussy about a clean litter box, he may be scared or ill. When a cat who was using a certain box before stops using that box, they are trying to tell us something.
I know this doesn't help much, but is just what I have run into in the past..
Good luck
M
Yes I take care of him for a friend and im taking care of him for 3 weeks. He was fine the first time i brought him home but this time he just wont use the litter box.
He might be suffering from the kitty form of irritable bowel disease, or reacting to new food. Have there been any environmental or dietary changes lately, or a new resident in the house that he might not like?