It's important to remember hydration is more than just water. Sweating means you lose salts and no amount of water you drink will help, you'll just urinate it out to keep your electrolytes balanced.
So it's quite probable his decreased urination was from being able to retain more water due to the salt intake replacing what had been lost.
So it's quite probable his decreased urination was from being able to retain more water due to the salt intake replacing what had been lost.