I would take the dog to the vet to make sure something isn’t going on that has a medical reason, first. It is true that older dogs have a harder time holding it. I would request a physical, including blood work to make sure that he’s okay. If the vet doesn’t find any explanation for this, then go back to square one with the house training. He could be going through an emotional issue. Has anything changed for him recently, however slight it may seem to you?