Thanks for placing your query here.
I can appreciate your concerns.
The boy seems to be suffering from viral gastroenteritis
Headache,tummy pain & legpains are most likely due to viral infection of the gut.
Can use paracetamol
syp in appropriate doses for pains .
Also please make sure he is drinking enough ORS
(oral rehydrationg solution).
If not better in 48 hrs or so ,please see a doctor.
Hope I have answered your query.
If you have any further questions I will be happy to help.
Wish him speedy recovery.