Horrible Nausea and Vomiting for weeks once a year

For the past few years, my boyfriend has had a horrible bout of an undiagnosed stomach issue. About every 8 months, he will start to vomit in the mornings, but will be fine for the rest of the day. Then, about a week in he will not be able to keep any food down, has constant nausea, terrible sweating, none of which subsides. He's had a colonoscopy, an endoscopy, cat scan, etc, but no one can seem to figure out whats wrong with him. The first time it happened it lasted for 2 weeks. The next time 6 weeks and he's now going through the third round. Then it suddenly goes away and he's fine. We are at our wits end here. Its so hard on both of us. Anyone have anything similar??