Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
It is hard to know , considering your guys age, if the diarrhea is the primary cause or secondary to something else. The first thing you should do is have your vet perform a thorough physical exam and blood work to rule out conditions such as Kidney or liver disease and many others. If these are normal, follow these instructions:
Fast your guy for 12-24 hrs to allow the digestive tract to calm down. Water is ok.
Give Imodium AD at ½ caplet per 15 pounds of weight every 8 hrs.
After the fast, feed a meal of rice and boiled chicken
If this has resolved, wean your guy onto regular food. If this persists, have your vet examine your dog. Even the dehydration from this can be serious and then your vet will determine the cause so treatment can be initiated correctly.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, ***** *****