Short answer to your question of whether it's appropriate at the high school level: no. Long answer:
As a life-long baseball fan, and someone who's played the sport for many years (including at the high school level), my take is there's too much showboating and not enough sportsmanship in
all sports -- and at every level. That all being said, there have always been certain unwritten 'codes of conduct' in baseball; one is that you don't show up the opposing pitcher if you homer off of him.
Frankly, it's kind-of hard to blame kids at the high school level for showboating when the professional levels have been increasingly doing it for years now. Look at old footage of power hitters from the 70's and before; when they hit a home run they just trotted around the bases and his team congratulated him at the dugout. There are exceptions of understandable displays of jubilance (in chronological order):
Bobby Thompson,
Bill Mazeroski,
Carlton Fisk,
Joe Carter; there are probably others I'm not remembering at the moment... But even Kurt Gibson was such a professional that he barely expressed any jubilance as he rounded the bases following
his dramatic walk-off home run in Game 1 of the 1988 World Series. But these days, routine home runs are exclamated by bat flips, batters who linger in the batter's box to admire their accomplishment, etc. I'm more and more reminded of
this scene from the movie Bull Durham...
Which brings me to your other question ("How would you feel if it was your son that gets hit?"): Frankly, my son wouldn't be showboating in the first place; but if he did so inappropriately, I would recognize that he violated the 'code' and accept the consequences so long as no one is seriously injured. Frankly, it's to the pitcher's credit that he chose not to throw at the batter, even after being encouraged to do so by his coach. If anyone's to blame in this instance it's both coaches: one for encouraging a retaliation at the high school level, and the other for not publicly admonishing his batter for showing up an opposing pitcher at
any level.