Thank you for your query on Healthcare Magic.
Feeding an 18 months old child is a difficult task for parents as their appetite is low in comparison to the previous months and they develop autonomy so if you try to feed them, child refuses.
So first thing parents should have so much patients to feed the child.
Just milk is not sufficient for the child growth and activity at this age.
Try to give more solid food and make them more calorie dense by adding some ghee or edible oil.
Don't give anything before his regular meal time.
Encourage him to feed himself with a spoon in addition to feeding by mother.
should be treated by addition of an iron syrup in a prescribed dosage.
For appetite increase you can give an oral multivitamin
preparation after consulting your doctor.
His weight is ok, but a little bit on lower side of normal range for his age.
Hope I have answered your query.