I would go ahead and switch him over to a diet of only boiled chicken and white rice or cottage cheese. Both of these are bland and they will help to settle the stomach. Feed only this. No dog food, treats, or anything else.
Then see your vet (or a different one!) for an exam. I don't believe that this is because of age. There is always a cause, even with issues that are age related.
Some ideas might be pancreatic insufficiencies or any number of other causes.
Let me know if you want to discuss this more.