What do the vets say about his weight and build? If his assesment is fine then there may be no reason to worry. My friend has a little guy who is 3years old and weighs only 4 1/2 lbs. He only eats food one piece at a time. He does have access to food all day long though for that reason. In my opinion, as far as the vet is concerned, you really should settle with one. You need to develop a relationship with a vet. Bouncing around is not going to help you or your baby. Settle on a vet and after you have seen them once or twice restate your concerns about his eating habits. Believe it or not vet offices do talk to each other so you could be developing a name of being paranoid. Just trust your vet to do the right thing the same way you do your family doctor.
Trevor