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Your child is underweight by just 2 kg. You should try giving different food items and try new recipes.
All kids are fussy eaters. We should put our maximum effort to make them eat.
You can give multivitamin syrup like Visyneral Z Syrup (vitamin A, vitamin B1, vitamin B2, vitamin B3, vitamin B5, vitamin B6, vitamin C, vitamin D and zinc) 5 mL once daily. But, this should be in addition to the food and it is not a substitute for food.
Make him eat fruits like papaya, banana and make him drink loads of water which will relieve him from constipation.
If his stools are still hard, give syrup Looz (Lactulose) 10 mL twice daily until his stools are soft. Consult your specialist doctor, discuss with him or her and with their consent take the above medicines.
For more information consult a paediatrician online --> https://www.icliniq.com/ask-a-doctor-online/paediatrician