I would go back to the vet. Clean the area very very well with natures miricle or a similar product.

Also are you sure it is just him that urinating? If they are both free roam & only do it when you are asleep maybe they are both ding it? Maybe she smells his urine so she goes there, then he does the same, over & over again.

Also once you have gone back to the vet & you clean the area really well maybe to be on the safe side also place something in that spot, a piece of furniture or something.

Have you switched his food recently? What type fo food do you feed.