I am wondering how my son gets such good velocity and movement on his pitches when his arm action and delivery are the same and only his grip changes. I looked at his videos when he was 10 and noticed when he finishes his delivery his thumb points to the ground and his palm and elbow faces out when he finished his pitches.
This video, taken this summer when he was 10 and warming up for a game, is when he was throwing 99% 2-seamers. His 2-seamer tails in on the RH hitter. By changing his grip it acts as a slider and tails away from the RH hitter, and is the perfect pitch against the LH hitter. He used the slider grip (he said exclusively 'cause he wanted to see what would happen) for an inning (under 20 pitches) the first time recently and induced four consecutive grounders to the heart of a very good line-up.
Does he show signs of high-speed supination? If so, how concerned should I be? This is his natural throwing action and probably is how he threw the ball when he was 4.