What the FAA controller did was in poor taste and ill advised but lets put a different slant on it.
He was standing there, in all probability, with a headset on and microphone in his hand, ready to interupt at any moment. His son was getting the thrill of a lifetime, possibly charting the future course of his life. I have heard the transmission by the young man. As a former pilot with over 6000 hours, I would have chuckled, said Roger that and been on my way. Remember, it is a pilot's perogative and duty to question any transmission which he considers inappropriate or suspect. He has the final authority and control of an aircraft and its operation. It was a mistake but not one that should cost someone their job or livelihood. If the same attitude had been shown me as some show this young man, I would never have been allowed to begin flying at the age of 14 and probably would not have made it a career. I understand where the father was coming from. I also am thankful my father had somewhat the same attitude.