Intense/bright sun can nearly LITERALLY paralyze my dad ANYTIME of day, even w a cap on. We practically sprint to the car/bldg for shelter. I don't understand my dad's reaction. He has fallen a few times (one incld major dislocation of upper jaw, requiring surgery) but never diagnosed with head injuries, although once was discovered passed out on sidewalk. He was apparently trying to escape hot sun, had locked his keys in car and was trying to get to the keys.
Has your Dad had any eye surgeries such as cataracts? Many people find that they can no longer deal with very bright light. My elderly parents couldn't. They got those eyeglasses that change to dark if in a bright setting.
As for passing out, sometimes being dehydrated can caused a variety of different health issues. Get Dad to drink more water and see if that can help.
Otherwise, as mentioned above, have Dad see an ophthalmologist.