Back foot slider... yep. He's had 3 pitching workouts in the last 10 days (on top of his core strength training, speed & agility training he does multiple times/week.) His trainer is VERY protective of pitching arms and none of the pitchers are throwing balls during the workouts. So none of the workouts have been with a ball. They have been with a glove in the left hand and a rag in the right hand - if you saw "Million Dollar Arm" it looks like the workout the players from India were doing at USC.
Son is a Center Fielder who has no problem throwing a rocket from CF to home so I'll be very curious to see what happens on an elevated mound once they start throwing pitches.
I sure appreciate all of the feedback!