Hi B.:
I am a pediatric nurse and I have to say that, as nurses, we are not allowed to diagnose anything. I haven't read through the other columns of advicem but want to say that mothers know their children the best of anyone - even doctors - and I would definitely take him to another pediatrician for a second consult.
My concern would be that since it seems he doesn't have fat to burn. When calories are limited, for whatever reason, the body loses muscle tissue. And of course, the heart is a muscle (problem with low carb diets, for example) Not trying to give you frightening info - just would advise you to continue to get other opinions.
I would also like to recommend something and will even send you a sample if you would like. My 12 year old is on these products (and I use them too) He takes an Isagenix protein shake with fruit as a smoothie at least once a day. This is a WHOLE FOOD so it can be used as a meal replacement. These shakes are undenatured protein (No heat is used in processing) and they are gluten free. Each shake/bar (also available in bars if he prefers that) has 23g protein, 28g carbsand tons of vitamins (I will be happy to send you all the info if you desire) My advice would be to get him some nutrition in any form he will take. These shakes can be made in 1/2 quantities and they are wonderful with fruit in them.
If you would like more info, please contact me. I can even assist you in getting them wholesale if you are interested. (The company/products are backed by medical doctors, scientists and nutritionists - you may want to do some research at www.infoisagenix.com to check it out)
You are a good mother - trust your motherly instincts about him and sadly, you have to often push medical professionals. Best of luck - I will keep you both in my prayers!
Blessings,
E., RN