Weight should be according to height and age .
For adolescent 10- 19 weight should be normally 4×age =4×15 = 60 kg should be for your grandson.
So 25 kg is very low weight for your grand son .
Reason of low weight should be evaluate .
Next Steps
You should take consultation and tell the complete history of your grandson then correct reason will be found out and accordingly food and treatment will be given.
Health Tips
Please say to your grandson -
Don't take outside food and Please keep positive attitude to fight any disease and any problem of your life.