What a charming way to act and I'm sure, one that boy will remember even as an old man.

I recently did something similar, at least in a small way. We were at a cafe for a rare cup of coffee out. It was a special occasion and coffee is not normally in the budget. At the counter was a nervous new employee. He was so kind and so nervous, poor guy.
As he was totalling our order I noticed he was almost 10% off. I could have said nothing, paid 10% less and have saved on that rare treat. Instead, I told him and pointed out exactly what his mistake was. He got a little flustered and asked me why I would tell him that and I said that honesty gets you the farthest in life. (I left out that I knew he would have to make out the difference with his own paycheck). I also told him I had enjoyed being served by him and that he could relax, he was doing a great job. When I left he had a big thankful smile on his face and so did I. I think I gained as much from that experience as he did and those 10% were well spent.