If it was me, I'd out the tacos in the fridge and eat them the next day. I think he believes next-day tacos are worthless due to flavor and texture changes, or some vague philosophy of only eating the freshest fast food.

My girlfriend's son operates similarly; he often makes a box of macaroni and cheese for dinner, and only eats half of it. He refuses to eat left over macaroni, so regularly at least half the box is wasted. My suggestion would be to splits box in half prior to preparation, but so far that has yet to be impmemented.