I feel that focussing on the release point is barking up the wrong tree. Location/control comes from having repeatable, efficient mechanics and a body conditioned to support them. Then it is about "feel". Practice, practice, practice. My son had horrible control issues for many years. Threw quite hard (I think but never had him gunned) but couldn't throw strikes at all, let alone hit the corners. An old coach asked me one day what we need to do to get him to throw strikes. My answer was "get him to throw more" with the intent to hit targets. Throw more. Last season, we saw a remarkable difference. He practiced more and it paid off. He now paints the black instead of throwing plumbs down the middle.
The other thing is that a teenager's body is constantly changing and that changes the entire feel of things. They are always having to get used to a new body.
Work on getting efficient, repeatable mechanics. Condition the body to support those mechanics. And pitch a lot with targets in mind.