Would he stop harping on this if you took them to the dr and had a blood iron test done? They are pretty simple finger-prick tests.
You can't access the full text of this article w/out a subscription, but you can get the general gist of it from the beginning part that you can access free
here . (From 2004 Pediatrics in Review -- article entitled Vegan Diets in Infants, Children, and Adolescents.) You may also be able to get the full text from a university library, if you have one close to you.