The muscular coordinatoin and sequence timing of each “control” within a muscle group takes into account how – quality wise, a pitcher’s repitoure, is mastered. Only after years of hard work, not forgeting muscle maturity, do we see the finished product, and at that, still a pitcher is like a work-in-progress as time goes, he gets older, experiences injuries – heals, rehabs, and so on.
I would venture to say that your son, at ten (10) has none of the musclue maturity mandates to show a difference in his pitch selection. I would also venture to say if he did have some variation in any of this pitches, down range, it would be by mere happenstance, without any real difinitive answer(s).
On the other hand, perhaps his methods of pitching could be reviewed by someone with experience of dealing with this age group and show him a few things that might give evidence of changing his velocity some, without actually “fluffing” the ball deliberately.
From all the lawn chair vantage points that I’ve witnessed last year and some of this year, for any ten (10) year old to just get it over the plate is enough to get-em dancing in the streets. Ten (10) year olds have a rough ride out there. I give your son credit for just being out there.
I know this doesn’t address your question head-on, but perhaps it may shed light on one aspect, of many answers, that your looking for.