How can I stop the dog from crapping on the floor?

Our dog, Digger, is a (circa) 15 year old Manchester Terrier. He was an abuse case rescued by my husband long before we met. Lately, however, he’s taken to using the floor as a toilet. He pees randomly, especially during dinner if we don’t feed him table scraps. Most recently, though, he’s starting to crap on the floor, most often right as soon as he comes in from being let out to do his business, almost as if he’s saving it for us. It’s not as though we don’t let him outdoors.

Digger is otherwise in fine health, and still able to romp and play, with a fine appetite. I know the answer is “take him to the vet,” but I was wondering if anyone here has been successful in convincing their incontinent old dog to stop doing their business indoors.