Doesn't matter if it's his son or not. The coach is WAY out of line and should be unemployed, and possibly unemployable as a coach going forward.
I had to testify in court in a similar incident 15 or so years ago. Coach/dad beat the crap out of his catcher/son in the dugout between innings after the catcher attempted a pick-off on 3rd and airmailed the ball into left field allowing two runs to score. I was the plate umpire. Mama bear had coach/dad on the ground begging for his life before we even realized anything was going on. Coach/dad was arrested on the spot.
The moral of the story, you cannot, under any circumstances, put hands on a kid!