I wish I had better advise to offer you, but I am in a similar situation. My daughter eats a slightly greater variety of foods, but she would absolutely starve herself if not getting preferred foods. Keep searching for answers
First, pay close attention to what the lab work indicates. The HCT/HGB (hematocrit and hemoglobin) refer to aspects of the blood work that the dcotor ordered. If all is well medically, you needn't worry. Of course, as a parent you are going to be concerned if your child does not eat in the manner you expect he should. But it's important not to struggle about this. Mother Nature, so to speak, will take care of this. Assuming he gets a clean bill of health from his doctor, just present the food, don't cater to him, and he will be fine.