Hello, first thing,he is not underweight 12 kg is avg. weight.
Constipation has different underlying causes.
Next Steps
consult properly with ayurvedic doctor nearby or online
Health Tips
1) increase water or liquid intake.
2) avoid dry veggies prefer gravy veggies.
3) give him desi ghee with milk or with rice or in any manner.
4) when he feel potty let him concentrate on it .
5) if he is not having any pain or other issues. then don't worry about constipation