Not saying definitively right or wrong but what he did in the playoff game was quite different than what he did in the earlier game. I could understand the logic behind the playoff game scenario, assuming your son's pitch count was low and his arm felt great. Hypothetical - I'll assume he is #1 guy. Regional playoff - win or season over. With five innings available, he cruises through three with low pitch count and team is still in the game. You are home team so someone else is going to have to eat two innings at some point and the team is going to have to scratch a few runs together. Would you rather be able to finish that game with #1 on the hill or someone else? And if #1 is still in the game making throws and staying warm, is this that terrible? (I know that some here would say yes). #2 guy was also efficient and game was low scoring so not much time elapsed between 4th and 6th innings.
Now the game where he pitched 1st, 3rd and 6th innings, I have no idea. There is also much to discuss with the starter going to ss after pitching but that's another discussion.
So, if the hypothetical circumstances were close to accurate, one could argue that he didn't lie and do the same thing. Still worth having a discussion between now and next season. Nothing wrong with talking to the coach when care of son's arm is in question.