What Can Cause Less Urination,less Sweating And Severe Headache?

What are the symptoms of heat exhaustion? My husband, who is not in good physical condition, worked outside today, and is feeling terrible tonight. He only urinated once all day and his urine was dark. He drank his normal 3 or 4 cups of coffee along with a few glasses of water, three bottles of Gatorade, and a glass of iced tea. I would have expected him to urinate more than once all day and his urine is never dark. Should he seek medical attention, or is there something he can do on his own? He also has a headache on the left side of his head that is not like a normal headache for him. It is not a terribly painful headache, just nagging. He is also questioning why he didn t sweat more today, since it was in the 90s with pretty high humidity.