Why Is My Cat Urinating On The Floor Instead Of His Litter Box? I Keep His Litter Box Clean And He Never Has Done This Before.

Many cats urinate any where and don't use litter box when they have some illness. If your cat is not neutered/spayed then this
can be another reason. Get him neutered/spayed.
To make use to of litter box again, do following.

1.  If the litter box is not clean then make
   habit of cleaning daily.
2.  Sometimes changing a litter box can work.
   
3. Change in the location of the litter box can also be the reason. Either put
   it on the previous location or change the permanent location of litter
   box to some where else where your cat likes.
4. Stress can be another reason.
   

By keeping in view above points try to train him. That might work. For medical cause, take her to vet.

Try to give your cat a tour of the bathroom and at least show him/her the litter box 2-3 times and try not to give any higher than half of the water and food and keep his/her catbox clean