What Could Headache, Loss Of Peripheral Vision In Children Suggest?

My 16 year old son started having sudden headaches followed by loss of his peripheral vision, last about 20 minutes or so. It does not seem to have a set pattern, he could be warming up for track practice, or sitting in front of the computer. He also seems to have a bit of mental confusion afterwards. Any clues, should I be overly worried?? Please inform!!