13mo boy vomited for two days. flu? then symptom free for 36 hours. has started vomiting again in large amounts. is it normal for flu to come back?

Unusual: It is unusual to be completely symptom free for over a day and then have symptoms come back as bad, or worse than they were before. I would recommend having your son checked out by his doctor to see if there is something other than the stomach flu causing his symptoms.

Not unusual: As the child starts to re-feed after a bout of vomiting, it is not unusual for the stomach/intestine to revolt and say "too much too soon" and the vomiting recurs. Take a step back to tiny sips of clear liquids such as pedialyte, monitor urine output, and if vomiting doesn't settle down in next 12-18 hours, needs to be seen by his doc.