Hi,
I understand your frustration, and I don't want to add to it, but I come from a family that has a lot of intestinal problems and some of the symptoms that you described could be something more than him bein finicky.
My son is 6 and he cannot gain weight if his life depended on it. I know why. I myself have Chron's Colitis and IBS that did not get diagnosed until I was over 20. I was very finicky because some things I would eat would hurt my stomach, or my stomach was nauseated to begin with. I have struggled with my weight my whole life and now, at 36 I weigh only 101 and that is not by choice.
My son has not been tested for any intestinal problems because the tests are quite invasive and he is too young for them. We just try to feed him as much as he wants of the foods he wants to eat.
My suggestion is maybe take him to an allergist. Some of his foods he may be allergic to, or he might even have a gluten intolerance (allergy to main component of wheat). If you can rule out foods he is allergic too, his body might be able to then digest other foods. Most common allergies that cause stomach problems: milk, gluten, msg (monsodiumglutamte).
Also, be careful if he is on pedisure, because that is made mostly with milk, and if he is allergic or intolerant to milk it will cause his intestine to kick it right through without absorbing anything from it and make sure you look for diahrea (sp?).
Good luck, and for your and his sake, I hope he is just finicky, but I wanted to let you know that there may be an underlying cause. If you have any questions, you can e-mail me at ____@____.com
Good luck.
L.