Your son has something that almost no one who has posted a clip on this website possesses..... he knows how to throw the ball ( arm action). This is also validated by his velocity (8th grader throwing 75 mph).
I wouldn't change a thing right now, especially anything having to do with the pivot foot.
For someone who has not done a lot of pitching I would suggest having him work out a stretch until he becomes more comfortable on the mound (game situations).
Other than that I wouldn't change a thing, at least not until you get some videotape of him throwing under game conditions.
I will also say that the quickest way to potentially screw up your son's pitching future is to worry too much about making him a pitcher at this stage of his development. And more specifically listening to any Tom, Dick or Chris's opinion as to what you should or should not do.
as a set of all the players that I've viewed posted on this website your son has some of the best throwing abilities.
As usual my not so humble opinions. :evil: