No need to worry. Nowadays this is a common issue, because nowadays kids are hyperactive and they are not taking food properly. And boys are generally under weight in their first five years. And your kids are not that much under weight the normal weight is 12.5kg and your kid weight is 9.5kg.
Next Steps
Give his home made balanced diet.
Health Tips
Don't introduce package food, instead of package food you can give home made food or snacks like instead of package chips you can give popcorn or makhana instead of chocolate you can introduce Jaggery and dry fruits ladu, instead of package cereal you can give some home made porridge or khichdi.