Answer
Dear Mrs Asif,
Get your son food allergy test done. Maybe he is lactose intolerant?! Also, after the results are out:
Try to give him, his friendly kind of diet from the list below:
Vegetables and fruit
Protein like egg poultry fish beef mutton beans and pulses
Starchy carbs and whole grains
Fats like nuts seeds olive oil butter and coconut oil
Water
Dairy
Also fussy eaters should be given multi vitamins incase of any deficiencies.
All the best