The goal of prescription food is to stabilize the dogs, hopefully stop the vomiting and diarrhea. Then, if and when he is doing well for a few months you can talk to your vet about the possibility of transitioning him to another food.
Senior dogs often do best on a simple, bland diet. Such as the appropriate therapeutic diet.
I think some of your confusion is because you are listening to homeopathic vets vs traditional vets.
Views, opinions, treatment differ greatly and often interfere with each other.
Best of luck
