Hi and welcome to the board!
The change in food might the reason for your dog's symptoms, but it could be coincidental. When was the last time he had a vet check?
You need to take him in for a complete checkup, since there are many medical problems that could cause what you are seeing - the vomiting, lack of appetite, etc.
If there is nothinng physically wrong, then change him back to what he was eating before, since he is 14 years old and should have what he feels good with eating.