Hi welcome to H.C.M.Forum. it is an infection of the intestines.called foodpoisoning. if it is not promptly treated in time it may leed to diarrhoea
. i advise you to consult a pediatrition for diagnosis & treatment.
besides you give him dilute milk, curd rice, dilute poridge.
dont give him fruits, nonvegitarian, & outside foods for few days. dont worry he will be fine.