Our sons 10 year old dog has started pooping in the house. He has never done this since he was housebroken at 2 months old.

Since he is 10 I would say maybe it is that he is getting old

The only things I know is have a regular schedule -feeding and going out (plus put him out when he seems to need to go)
Have a spot to train him to use the piddle papers when he can not make to the door. Him being older his joints might hurt too much to make it to the door.