What are you feeding him? My dog that has a sensitive stomach does well on Nutrisca Salmon and Chickpea as a base, no supplements except a fish oil capsule every day.
You could try presoaking the food thereby making it easier to digest.
Has he had a senior workup recently (labs etc)? There could be something else going on…..
I lost a corgi at that age due to an aggressive form of cancer that showed up suddenly without warning.
Some Science-based Veterinary Medicine here: http://skeptvet.com/Blog/?s=probiotics
If you are interested.
