My pd diagnose that my 5 month old is lack of iron, he is currently taking iron supplement, does it help him in long term basis?

Iron: Milk is a poor source of iron. Supplements are usually recommended. It does take time to start improving the hemoglobin. Around 3 months.

Yes: It has been demonstrated that children who have chronic iron deficiency anemia can be sometimes delayed from a neurodevelopmental standpoint. Correcting iron deficiency and the anemia that accompanies it it a very helpful thing for the long term health of a child.