doctorwang:He is trained to do his duties outside, but recently he is pooping just in this one area of the house.
Dogs don't generalize well. "Don't poop in the living room" doesn't necessarily mean "don't poop in the bedroom".
Each time a dog is allowed into a new area of the home, he should be fed there for a while and a bed should be placed there for a while. Your dog was given too much freedom to soon and did not develop a prohibition against pooping in that one area.
I would block off that area, clean it very well, and wait two months to help break the habit. Then I would slowly allow access to more and more of the area. Each time move the food bowl and the bed to the newest area.
Now and until you have finished working on developing a prohibition against pooping in that area, be sure to frequently and enthusiastically reward the dog for pooping outside. Use his favorite treat and, while you are doing this training, don't give that treat for anything else.