Definitely don’t want a loop diuretic. They inhibiting reabsorption of sodium of electrolytes.
In all honesty if he’s peeled and looks good I wouldn’t change much. I’d pull carbs down slightly not completely deplete and replace those calories with fats this will fill triglycerides stores in the muscle and still allow you to deplete glycogen until thurs or Friday assuming his show is Saturday then start adding carbs back in and pull water back a little bit to make sure he’s dry. If he drinks 2gal a day pull it back to 1gal on Friday. I wouldn’t use any form of diuretic if you don’t know exactly what you’re doing or how the medications work.