Housebreaking an adult dog.?

I got a 3 yr old male dog from a shelter. At first he urinated in the house to mark his territory. That has stopped now, but I take him for long walks and he seldom poos outside. I bring him in and within about 20 minutes he goes into another room and poos before I can catch him. I've tried locking him in the same room I'm in and he go off in a corner and poos. What can I do?