Exactly...he needs a nutritionist who can tell him what he, in particular, is losing when he plays, and when he sweats a lot. And my understanding is that they can fine-tune that very well these days.
Yes, everyone carbo-loads the night before a marathon, and drinks loads of water, but my point is that if you haven't been tending to that, all along, the night before is too late. Let me get a little nerdy here, for the laymen (not you): Your muscles and liver store glycogen (sugar) to burn as fast fuel. You can increase the storage space in your muscles, for glycogen, by stuffing them with carbs, then depleting them in training, then stuffing them again, and they "inflate," like a balloon. Feed them and deflate them, and they will hold more and more ready-fuel to burn. This all takes time, and is part of what training is about. When you've used up your glycogen stores, your body starts burning fat, and then muscle.
When it starts burning muscle, that's where you get damage, albeit generally short-term. (To @Front242's concern.)
Another thing that takes time is proper hydration. That happens over time, not overnight. Your body only learns to store water effectively when you teach it to.
I know that marathoning isn't the same as playing tennis, but, having heard the expression in terms of the Majors, "it's not a sprint, it's a marathon," I have to think that similar rules apply to the body of a tennis player.