I worked at an animal hospital that dealt strictly with cats, and this is a common problem. I am glad you had your cat checked out by the vet. That is the first step.
The second step is to realize that even though you perceive that nothing has changed, your cat has! Something might of scared him while he was in the litter box and now he has bad associations with pooping there. The first thing to do is add a few more litter boxes around the house, especially if there is a particular place he likes to poop. The next thing to consider is to put different litter in some of the new boxes. He may find one he likes better.
Also, if your box is covered, you should try taking the lid off. We saw lots of problems solved when the covers were removed.
Next, is to give him some time to decide if there is a box he likes better. If he poops in one and pees in the other, then keep them both. Something has upset him about the fist box.
And then, if there is no change, I would take him back to the vet for another opinion. Your problem is most likely behavioral, but you never know.
Good luck, and have patience. I know its incredibly frustrating to have a pet pooping all over the house. (I have found Simple Solution to be the best pet stain/odor remover)
Have patience!