Welcome to HealthcareMagic forum.
Your son most probably has a virulent gastroenteritis
which might not have resolved with Zofran though it is an appropriate treatment.
The reason could be various like resistant bacteria or viral infection or low immunity etc.
As of now he definitely needs to be reevaluated and investigated by his pediatrician
( stool culture test , CBC , ultrasound
These can confirm the diagnosis and then appropriate treatment can be started like alternative antibiotics ( Cefixime
, colistin etc) and zinc and probiotic
Do maintain his hydration by giving plenty of fluids and oral rehydration solution.
Hope I have answered your query.
Wishing your son a speedy recovery.