Low Blood Pressure Help Fast Please !!!!!!!!!!!!

When my dad is sometimes driving the car, he has to take it on the side because he is feeling some dizzyness in his head, he also fell down for 5 minutes 1 day, and another day he fell down 5 times!
We went to many doctors and they all say that his blood pressure is normal, but still these things are happening to him. Some doctors have said that he gets low blood pressure sometimes, and that blood is not reaching his head which makes him dizzy and which makes him fall down for a couple minutes. My dad only has low blood pressure or dizzyness when he donates blood. The other day he donated 500ml of blood and that\'s the day he fell down 5 times! Also my dad works for very long periods and also donates lots of blood. Tweet