I think I would be on the fence about this as some of the others were, however your son is 13. If he were younger, I might be able to see how this mistake happened. But, 13 is plenty old enough to read signs and listen to one's mother. At the Subway's that I have driven by, there are rather large signs advertising the $5 footlongs. So, unless you were unclear when you conveyed to your son what to order, I don't understand how this mistake occurred at all.

If I had to guess why the employee didn't speak up and say something to the boy, it would be that they are told not to. What would the advantage be to Subway be if they spoke up every time someone ordered a 6" and told them for the same price they could order a footlong? Subway is making money by bringing people in their door with the $5 deal and then selling them not only footlongs but 6" subs as well.