I wonder if he's like me and just doesn't 'get' thirsty?
I know that I have to be careful in places with high heat and humidity , and I usually tell everyone I'm with to remind me to drink, because it just doesn't cross my mind and I don't feel thirst. But I am aware that this could cause me to become extremely dehydrated, whereupon symptoms of confusion could set in without my even knowing that I AM dehydrated.
KNOWING what can happen is very different to actually HAVING it happen, because once you are disorientated then being dehydrated is the furthest thing from your mind. So he might not even have thought he needed to stop for a drink - when, in fact, it was the very thing he SHOULD have done.