Thanks for your query,
in your son is usually viral in origin.as you said he had seizure
associated with it,single episode for which i will advise to get pediatric consultation.
Regarding widal test
it is positive if To titre is more than 160 in active infection and TH more than 160 in previous infection or immunised individual,
I advise you to give him plenty of fluids,fruit juices,give him balanced diet,avoid fried food and all kind of junk food
Wishing your son good health.