How can I get my adult dog to stop peeing and pooping inside?

Please ignore all of the other comments about the dog trying to "dominate." I know it's frustrating, I just went through the same thing with my dog. He would have been 11 this October, but he was just put to sleep because of a tumor. He began having problems going to the bathroom in the house after being housetrained for 9 1/2+ years! Harley would go in the house sometimes after being in for only 15 minutes. My bf tried yelling, smacking his nose with a paper, and we finally realized the poor guy was getting older and probably couldn't help it. He was kept outside during the day, and let out frequently when we were at home at night. Just have patience with him.