High protein diet. And yes vitamin C rich fruits, dry fruits , dates for iron.
Give him the regular food u elders have. We take usual food, Ask him what he likes, he will say. 4,5 yr old will b school going age. So they know,from the other children s tiffin boxes,what they like or not. My son is 28 yrs old. Only now,i knew,he used to ve his friend s tiffin when he was in school. & he in return takes his. So give him what he likes. Along with some nutritious foods also,if he likes. or compel him. Making some tricks. Like taking out,or showing tv erc etc.