ETA:
He's had allergy testing?
You don't mention if you kept logs of when/how often it happens. I am assuming you do after 5 years.
My husband had abdominal/nausea type migraines as a child - not always with pain. Migraines can be 'silent' (no pain present). You consulted with neurologists - did they mention this? They have preventative meds - sometimes they put people on these to see if symptoms clear up.
You could keep a log to see if there were any triggers before his nausea. There are some well known migraine triggers. You could try eliminating them one by one to see if his situation improves.
My only other thought is nausea is often from psychological stress. All this testing etc. may have caused him duress - and if he had anxiety then has he seen a counselor? That may help. One of my children had a bought of anxiety and her only symptom was feeling an upset tummy.
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Are you asking what we think it might be based on our experiences? The website has been acting up today so maybe you got cut off.
Migraines, vision issues, inner ear (vertigo), constipation, anxiety ... come to mind.