Is the dog constipated or does he have soft stool? Metamucil and pumpkin have opposite effects- pumpkin hardens stool and metamucil softens it.
I would definitely not do what the vet suggested about caging and starving the dog. No way.
Have you tried canned foods? I know Lucky asked already...
But seriously, Merrick canned food is almost better than human food. You might want to try it out before trying any drastic measures (like starving, etc).