Our expert says:
May I ask how you know he dehydrates quickly? Reason I ask is that there's not a whole lot of evidence that dehydration is all that harmful during exercise. It's pretty normal to lose 3 ro 4% of your body weight during exercise, and so if that is happening, it's not actually a problem.
So what symptoms does he have to make you suggest it is dehydration? It may be a lack of energy if all he uses is water - perhaps you can consider something like Powerade or Energade, which is really good enough, unless we're talking multi-day stage race events. Otherwise, those drinks will work, and all he need do is drink to thirst!
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