I suggest feeding him something containing Vit C before giving him the iron. Then put the iron med into something he likes, non dairy if it hinders effectiveness of medication.
Perhaps he'll swallow the med with water and rewarded with a popcyle or sometHong he loves.
Does he take a pill or liquid? My grandaughter started swallowing a pill before she was five. If it's a liquid miss it into applesauce or anything he likes. You may be able to crush the pill and mix it in food.
At 5, ask for his help in finding a way to take tge medicine. Never hold down a child to take a medicine unless not taking it is life threatening. A 5 yo can agree to taking the medicine if he understands it's important and given a choice.
Yes, do call the pharmacist for help in knowing how to manage this.