I.C. gave you some excellent ideas...
And while I agree that magnesium oil (magnesium chloride) does work in some cases better than Epsom salt, I believe that I am chloride sensitive - when I use a mag\chloride it takes me a few days to recover... I feel even more run down. Halogens, such as chloride, fluoride, and bromide (iodine too - but they do not use this halogen much) act as delivery agents for whatever they tend to be bound to... that is why they are used in pharmaceuticals.
Mag oil may work great for you though given the circumstances and heat related issues. Just pay attention if you go that route.
Switchel is a drink that has been around since ancient Sumeria... it is made a number of ways and was used to prevent the very thing you are describing...
It was generally made with a naturally, raw vinegar, or lemon (lime, orange, grapefruit can work in a pinch too) juice, and a raw, whole natural sweetener such as honey, molasses, or maple syrup.
http://www.bing.com/search?q=switchel&form=OSDSRC
Find what he may like taste wise best - I like lemon or lime and honey - a honey lemon\limeade with a bit of mint.
He could take it to work and sip on it all day...
If you feel that more electrolytes are needed, a bit of whole, unrefined salt can be added... not too much - you do not even have to taste the salt for it to be effective.
Obviously some of the ingredients listed would be more mineral rich than others, and thus more efficacious, however, it is better to find something you like the taste of and will drink IMO.
As IC stated, if you drink distilled and/or R/O water, you should really try to restructure it in some way.
The switchel will do this.
grz-