First, was it just a finger stick in the office, or did they send blood out to be tested at a laboratory? if just a finger stick, I would ask for a regular blood test first, as the hemocue machines (the in office ones) are notoriously inaccurate. They just provide a range, and sometimes it is operator error ( if not put in the machine within 5 minutes, not a big enough drop of blood, etc) they can get a false reading that was as well.
Second, make sure you are aware that iron supplementation can lead to constipation, so change his diet accordingly once you start the supplement (if you do)
Third Have you preiously been giving your baby iron rich foods?
winter squash
sweet potatoes
prune juice
meat & poultry (beef, beef & chicken liver, turkey, chicken)
mushrooms
sea vegetables (arame, dulse), algaes (spirulina), kelp
greens (spinach, chard, dandelion, beet, nettle, parsley, watercress)
yellow dock root
grains (millet, brown rice, amaranth, quinoa, breads with these grains)
blackstrap molasses (try adding a little to cereal or rice)
brewer's yeast
if not, maybe ask the doctor to allow you to try dietary changes with a retest at the next visit? cooking on an iron skillet/iron wok is still recommended from what I remember, as well.
It all depends on how low the level was, etc. Have a talk with the ped about it and discuss your options if you are not comfortable with the supplement.
Good luck!