If he is that thirsty, he's dehydrated
Is his output high? How many times does he empty and is it always liquid?
if he's drinking high sugar drinks (Gatorade is and not sure about
the muscle milk) they will cause dehydration...which makes him constantly thirsty. Chinese uses MSG which will also cause diarrhea/dehydration.
Does he have instructions for Imodium and Metamucil? Imodium will slow transit down and the Metamucil will bulk up the output (fiber). The BRAT (Bananas/Rice/Applesauce/Toast) diet will help thicken up, too, you can google it for specifics.
He may need to get an IV with Potassium to catch up and start fresh from there.
Salty Pretzels/Peanut Butter Crackers/Saltines and other similar foods will help retain water which will help him feel better, too. Mashed Potatoes are great (think Potato Buds) for thickening and one of my go to foods.