Tell him that X * 25% + Y * 25% = (X + Y) * 25%. Unless you think he is a Hanes wearing man’s man who will grab your Fruit of the Looms and give you an atomic wedgie.

Tell him, “You’re a problem” and take the food away from him saying “No soup* for you! One year!” *or whatever the food actually is.

Unless there is a restriction which says he may only use one coupon per purchase he’s almost right. 25% off R100 is R25. A second 25% is another R25. However 25% off the remaining R75 is R18,75. So the total discount for using two 25% coupons for the same purchase would be R43,75 which is a 43.75% discount.

You can't fix stupid.