All of the elite pitchers keep their back foot on the ground until after the ball has been released. What this indicates is that they are keeping their upper body more upright through release. Pitchers who lift their foot off the ground too quickly are getting too far out in front with their upper body. This means they don't fully use their hip and shoulder rotation to throw the ball - they throw with the arm more than they need to. If your son is doing this then it certainly does mean he could be "leaving some velocity on the table". But, more importantly, this could cause some extra wear and tear on the arm that might lead to an injury.
The cause of the foot lifting off the ground too soon is a lack of flexibility and/or functional strength in the lower back. It takes flexibility to achieve the upright spine late in the delivery and it takes strength to maintain it up to release. When the shoulders square up to the target and the throwing arm lays back in external rotation, the lower back arches and holds an isometric load for a short period of time. About the time the throwing arm snaps forward, the lower back releases. If done in this sequence, then the shoulders have rotated around an upright spine and power is optimally transeferred through the chain of links from legs to hips to shoulders to arm to baseball.
The fact that your son's back foot dragged for 10" is an indication that he has a reasonably long stride - this is a good thing. But the fact that his foot drags that far and still lifts off the ground before the release of the ball (which seems strange to me) tells me that your son lacks lower back strength moreso than flexibility. That is, he apparently has enough flexibility to initiate an upright torso but lacks the strength to maintain it all the way to release.