"(A)t a young age 8,9,10 his fastball looked like a slider dot in the center."
This is the most telling fact, in my opinion. I would bet a non-trivial amount of money that what your son is doing is throwing an involuntary (or not) slider some of the time that he thinks/says he is throwing a fastball. Does your son's fastball seem especially wicked compared to those of others? Does it move more? Is it harder for catchers to catch? Is he a great strike-out, ground-out pitcher even though he doesn't always keep the ball down? If so, then that suggests that the ball is diving right at the end of its flight.
"(I)ll check dr marshall web."
Be warned that he is undoubtedly the most opinionated person you will ever meet. He also thinks he's smarter than everyone else. This rubs a lot of people the wrong way, but I work with programmers so it's just par for the course for me. Of course, having a PHd and a Cy Young will do that for you.