9 month old is anemic even though he’s already taking a vitamin?

I just had my son’s 9 month appointment and the nurse tested for anemia. She said his levels were fine and sent us on our way, but the doctor called that night and said they were actually 10.6 and supposed to be above 11. He called in a prescription vitamin with iron, but I’ve already been giving him an over the counter vitamin with 100% DV iron. He’s also formula fed and there’s iron in his formula (Similac PA) and he’s currently getting 30oz. He also eats a ton of Cheerios…I told the doctor all of this and he said they’d check again at the 12 month appointment but the more I think about it, the more concerned I feel. He’s nearly 24lbs so he’s definitely growing very rapidly, but he theoretically should be getting iron from a ton of sources. Has anyone else experienced this before? I know anemia is common but I can’t imagine how low his levels would be if he weren’t already on this OTC vitamin!Edit to add: he also gobbles all solids presented, we have no issues with getting him to eat whatever we present via /r/Parenting https://ift.tt/2UE50Rp