I went with a friend to a lecture on ADD/ADHD, and they were saying that the kids commonly have a whole range of vitamin and mineral deficiencies, and many have blood glucose assimilation issues (hypoglycemia, hyperglycemia, etc). This was not an MD seminar, but a holistic/naturopathic one. If that turns you off, you probably need read no further. However, in case you are interested, this particular RNP was saying that she has had an 85% success rate treating with diet, with "success" defined as the parents reporting marked relief from symptoms. The best news about it, to me, is that taking vitamins and minerals in no way interferes with meds. She said that she does not ask her patients' parents to take them off meds, or in any way go against their doctor's advice. She just provides nutritional information and support, and the patient/parent and doctor can go from there and decide what is or is not working, and what is best for the child. I remember the iron being mentioned, also the B vitamins, and some specific amino acids and things that I do not remember. If you have naturopathic care available in your area, it may be worth it to you to have a visit and see what they say and what you think. It can't hurt anything.

Also, when my daughter was seeing a naturopath years ago (whole other story; un-diagnosed food allergies that made her an extremely ill child for the first five years of her life) Blue Cross paid for it since the practitioner is an RNP. They wouldn't pay for herbs/supplements, but those are often reasonably cheap anyhow. Just a suggestion, in case you are interested; your insurance may even pay.
On another note, my daughter also struggles with low iron, and we were told to cook in cast iron once or twice a week to help with that, along with feeding her beans and other foods high in iron. I'm told that watermelon is high in iron as well, but don't know that for a fact myself.
Best wishes, I hope you find the best solution possible for your son.