Try placing some of his poop in the litterbox to give him the hint. If that doesn't work, you will have to keep him out of the rest of your house until he starts using the box. Be sure to thoroughly clean up any poop messes, eliminating the odor as well. He might be going back to the place which smells the best to him. Might also try 2 litter boxes.
Some cats refuse to poop and pee in the same place.
Make sure the boxes get cleaned at least once daily. You might have adopted one of those cats who just will NOT step in a dirty box. My cat was doing this and the best thing to do is to keep them contained in an area with the box for a few days when you are not around. They get used to using the box this way. Also, make sure the cat is not feeling threatened in some way.