I know how you feel. My daughter was two months old and admitted to Children's Hospital for malnutrition. Why, not because I wasn't feeding her but because she had a protein allergy and was not able to tolerate the formula she was on. She was extremely underweight and had horrible digestive issues. THey sent me for testing for cystic fibrosis, cysts on her liver, on and on...what was it, a simple change in formula. Because of her malnutrition, my daughter contracted a bacterial infection in her gut, was in the hospital for 9 days and on IV antibiotics for 24 days. It was a nightmare and all the doctor had to do was switch her formula.
The GI Specialist at Children's said the pediatrician I go to is the one he recommends and one of the best in the state. So I decided to stay, because now they know my daughter's health and it would be better than starting over with a new one. But I had a very serious heart to heart with the doctor. I told them their ignorance bordered on malpractice and I was extremely disappointed in all their staff for shooting for the extreme diagnosis and missing the obvious. This seemed to work for me. They are very, very responsive to me and I get calls back from the doctor, not the nurse, when I call them.
I suggest talking with them, letting them know how you feel and that you are disappointed in their service. I think if you mention the mis-diagnosis as bordering malpractice they listen.
Good luck, but I believe your child's best advocate is you and if you believe your child is sick, never give up finding someone who will listen.