Ped GI question

I am looking for advice for my 5yo grandson. He is a healthy kid with no health problems. For the last year or so, he has had episodes of vomiting (generally with a couple months between) which always occur in the morning. He c/o a stomachache, is pale, cold, and clammy, and vomits 3-6 times over a couple hour period. He is lethargic. Then he quickly returns to normal and wants to eat. He never runs a fever and once it is over, he has no problems til the next episode.

After this had happened numerous times, my daughter-in-law took him to the doc in Dec. They did a xray and determined that he had stool backed up. He has been on Miralax since then and has a BM daily and the problem seemed to be fixed.

This week he had another episode which lasted longer (about 6 hours), he was vomiting frequently, and he became weak enough that his mom took him to the doc (she was concerned about dehydration - he could barely walk and was lethargic). The doc said he did not feel any stool by palpation and he did not think he was constipated. He said it was a virus which lasts 3-5 days.Within a hour of getting home from the doc, he returned to normal so obviously it was not a virus.

Any thoughts what might be causing this? What kind of tests might be useful? My daughter-in-law is concerned but doesn't want the doc to think she is just a hyper mom. (He knows that she is a peds nurse - she works with kids with long-term health issues - vents, etc.)