Be sure your dogs fecal is negative for worms and parasites. A home made bland diet made with 1/3 rice, 1/3 fresh vegetables such as green beans or peas and 1/3 chicken would be an ideal diet for your dog and would also help to clear your dog's diarrhea. Fresh vegetables are a source of fiber which help to slow down gut "transit time" of food so as to promote absorption of dietary nutrients and clear up diarrhea.
Check the FDA pet food list to be sure the dog food(s) you have and are using are not involved in any pet food recalls as many are tainted with E-coli and/or Salmonella bacteria, both of which cause canine diarrhea.
Having your vet run a senior dog blood and a urinalysis would help to rule in or out any internal health issues that your dog may be experiencing and offer a valid baseline of health from which to work.
Digestive enzymes and a probiotic supplement would be extremely helpful to re-balance your dogs digestive system and populate his "gut" with "friendly bacteria" to absorb dietary nutrients and clear up the diarrhea.
Tylan, a product not labelled for dogs works like a wonder to clear canine diarrhea generally in 24-72 hours.
Should you want to follow up on your dog with me, that would be fine.
Dr Carol Osborne, DVM