My daughter has suffered from Chronic Vomiting Syndrome, also referred to as abdominal migraines. She did not vomit as often as your son, or in response to stress, though. It would come upon her all of a sudden, usually at night, every three to four weeks. A few months on a very mild antidepressant seemed to do the trick, and she hasn't had an episode in about six months. My daughter's pediatrician caught this condition, but he consulted with a pediatric neurologist on the medication.
Like I said, it doesn't exactly sound like what your son has, but I thought I would tell you of this syndrome's existence.
In my personal opinion only (I have absolutely no medical training, just a lot of interest and reading), it sounds like he is vomiting whenever he is overcome with strong feelings. I don't think a child of 3-4 would be vomiting for attention or manipulation. (I mean, really, do YOU like to vomit? Ick!)
I think you should seek a consultation with a child psychologist, or at the least, a different pediatrician.
Good luck with your little guy.
Dear Ms. McMasters,
The referral to a pediatric gastroenterologist makes sense. It is important to rule out Chronic Vomiting Syndrome (also called Cyclic Vomiting Syndrome, Abdominal Migraines, Childhood Cyclic Vomiting, Periodic Syndrome). This condition, in addition to the vomiting, is typified by nausea, fatigue, motion sickness, abdominal pain, and sometimes dizziness. From what you note, your son's situation is characterized only by the vomiting. Also, the pattern of your son'e vomiting does not indicate Abdominal Migraines (because his vomiting seems clearly related to situational antecedents).
It is possible that he is displaying an anxiety reaction, partly related to excitement or to underlying concern about something different. A proper plan would be to first investigate (as you are doing) the posssible organic or strictly medical bases for the symptom, and then pursue the psychological/psychogenic possibilities, via a child mental health professional, if the organic causes are ruled out.