you are saying he won't eat vitamins? You state you are worried it's too much carbs, he may not be as healthy as he should be due to his eating. But substituting things until you find things etc., may be the only way to go for now. But I would still rule out any physical reason as well. Making sure his throat is fine for swallowing foods, that his taste buds are not swollen, no infections...things like that. Example: if he likes ice cream or shakes, make one with some the ensure, as this will at least give him his daily nutrient as well to grow. Do follow some of the tips on that site I had sent you.
Plus as the site stated as well:
Once the medical/dietary assessment has been completed, the doctor can determine whether or not your child has a serious feeding problem.
Fortunately, the majority of picky eaters grow normally and without nutritional deficiencies or imbalances. In these cases, the approach focuses on an individualized combination of practical measures and behavioral techniques.
Hope this helps Hun.
Plus there is gum that has vitamins in them, gummy candy with vitamins, plus your boost or ensure in a variety of flavors as well.