What Causes Dizzy Spells Leading To Headache And Tunnel Vision?

I have a 15 year old son who has been having dizzy spells which lead into headaches and sometimes tunnel vision & disorientation. His doctor has run MRI's, CAT scans, EEG, EKG, you name it and they all come back normal. He has low to normal bloodpressure and had these spells for almost a year now. He didn't have any this summer but was on bed rest for two lumbar discs with stress fractures. His doc cleared him for activities 3 weeks ago and he has had 3 days of football practice with dizzy spells yesterday and today while practicing.