You need to check his pulse - is it strong and regular, or are there missed beats. Also, his BP should be measured when he is upright and when he is on the bed. Numerous conditions can cause
dizziness, but the most relevant ones are rhythm disturbances of the heart, a
high blood pressure, migraine, heat-related dizziness,
dehydration and, in his case, because he has a brain-
vertebral column malformation, circulatory problems in the blood vessels around his lower brain.
He needs a proper evaluation by a good physician, and perhaps, tests to determine if any of the above problems are present.
Rarely, problems with the balancing organ of the
inner ear may also cause these symptoms. If he has any ear problem history, please take him to ENT as well.
Hope this helps without scaring you ... I am sure he will turn out to have a minor issue, most likely migraine.
Best wishes.
Dr. Taher