Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
Great questions and great to do some research. In a dog of his age, it is awfully suspicious he does have a sensitive system, perhaps even inflammatory bowel disease. If he is underweight and chronic diarrhea, then a medical problem such as pancreatic insufficiency is also possible (this is when the pancreas does not product the proper amount of digestive enzymes).
I assume he is on his monthly heart worm preventive and flea/tick control year round.
So, how should you and your veterinarian work through this? From a testing perspective if he was my patient I would recommend start by doing a screen for the treatable problems with some lab work that included a comprehensive stool check to the lab (look for all those "odd" parasites not covered with heart worm preventive) and general lab work (CBC, chemistry profile and urine check - this ensures no protein loss from the loose stool and no obvious organ problems or infection) and then a screening for pancreatic insufficiency (PLI test). If all this is clear then you have to make a decision if you want to have biopsies done (surgically or endoscopically) to try to diagnose the exact problem (many people do not elect to go this far).
After that information, doing a diet trial with a hypoallergenic or limited ingredient diet is excellent. There are some wonderful prescription diets for this problem or you can cook. If you cook, I recommend follow recipes as balanced by a board certified veterinary nutritionist such as the book by Donald Strombeck: Dog&Cat Diets: The Healthful Alternative. You would then do this trial for 8 weeks.
I also highly highly recommend start now on a probiotic supplement. Not all work well. Either use one of the two good ones made for dogs: Fortiflora or Proviable or use the VSL#3 for people which you can order online and then depending in his size is one-half to one packet daily (had amazing results with probiotic alone).
Finally, if all that does not work, then prescription medication may be needed. If he responds to metronidazole, then a low long-term dose may be needed. Or discussion of using a steroid.
If you feel your veterinarian has not already talked you through and offered all of the above, then a second opinion is really needed or ask for a referral to your closest internal medicine specialist. This is a serious issue that can be controlled, maybe cured if there is a treatable condition like an atypical parasite.
I am at a point I need to know what questions you have. We can continue our dialogue in this setting.
I hope that the information I provided has been helpful.
Please remember to select REPLY TO EXPERT if for any reason you need further clarification, have more questions, or were expecting a different type of answer.
My goal is to provide the best answer possible prior to you leaving a feedback rating.
If you received all the information you needed, then kindly submit a rating.