This IS odd! Is his litterbox in a high traffic area, where people are passing all the time? Cats don't like this, and if there is any commotion near the box, sometimes they'll go elsewhere.
If the box is in a private area, and there is nothing physically wrong with your cat, I would clean under the bed with something like Nature's Miracle, which removes odours, and I would lock the cat in a small room - like a bathroom - until he is retrained to use the box.
Having a second litterbox in another area may help also.