What Causes Dizziness And Intense Headaches?

My 13 year old son is a competitive athlete. He trains 26 hours a week. The past 3 days they have been doing a new specific excercise at the end of workout which involves running across a gym floor pushing a big mat as a weight. He has conditioned much harder than this in the past without any problems. He turns really green, feels dizzy, and gets a very intense headache all of a sudden right after. He said that it feels like his throat is closing right before all this happens. Is it just dehydration or something? He's claiming that he is drinking plenty of liquids all day long, or could it be something more serious that needs a Dr attention. If so, what direction should I point my dr in as far as tests?