Pain on left side under rib cage may be due to enlargement of spleen.It is enlarged in many conditions like Malaria,Typhoid,Blood disorders etc
He has to be investigated with Blood tests including peripheral smear,Ultrasound abdomen.
Till then give him Paracetamol
and Anti spasmodic like Dicyclomine
If you come back with reports,i will clarify further.
Wish him speedy recovery