Do you own a crate for the dog? Sounds cruel but dogs love them and in time think of them as "their" space to escape too. Also, when their young they can spend some time in them and they will not go while in there cause they will never "go" in their home. They might of course if he's never brought outside enough but that's only common sense. I have a five year old (dog
) who has the run of the house and still returns to his crate on a regular basis to just chill out. Ask your vet about crate training, he/she will strongly agree I'm sure.