He´s stopped using the box!

A cat´s sense of smell is highly sensitive. Although they return to the same place they have been before to eliminate their waste, they will find a new place if the old one smells too strong.
Plastic can retain odors; your cat can smell those odors even if you can´t. If Kitty has stopped using the box, even if you keep it clean, it could be that he can smell the odors from the box. You can try to clean the box with the Odor Removing formula from the other Tips sections, or you can try purchasing a new box. Remember that the cat can smell things you can´t so most cleaners will be too strong for your cat´s nose, causing him to turn away from the box.