What Causes Dizziness, Nausea, Vomiting And Sweating?

My boyfriend ate at a friend s house last Saturday afternoon, and then attended a concert Saturday night. He was wearing a sportcoat that was too warm for the 90 degree weather, and by the end of the evening Saturday night, he was dizzy and nauseated. He vomited a few times. I thought maybe he had just become overheated. He did not have a fever at all, but was sweating and skin was cool. Today, 3 days later, he is still somewhat dizzy, and occasionally nauseated, but has not thrown up anymore. He says he feels like he has no energy. The dizziness is a little bit better, but not gone. What could be causing this?