There can be other reasons besides the one I posted abover. A youngster’s natural endowments can help tremendously - or the lack thereof can be a handful, when trying to evaluate and help with what your son is doing. I’ve seen some youngsters in the very early stages of their baseball experience have considerable trouble with holding on to the ball all because their hand is small, or their grip is rather week. Size and strength along with other issues, like late motor skills development, below average body build for the age and the like can make your evaluation even tougher.
By the way, the adolescent is extremely difficult to evaluate when coaching pitchers. There are so many variables in the mix that I tip my cap to those that do coach this age group, and younger.
Below is another reason for the release problem that your son might have. When a pitcher extends his pitching arm too far back after breaking his hands, he creates a “scarecrow” kind of stance with his upper body. This extension - back with his pitching arm, has a whipping effect when the arm is brought around during the actual throwing/release phase of his delivery motion. This entire posture can become a habit with many that’s impossible to correct if lift unchecked for even a short time - like a month or so.
In my work, I would use the back hip to monitor one of many causes-n-effects of a pitcher’s pitching cycle. I should note here that this was my way of doing things. Not every pitching coach does this. In any event, I would use the back hip for upper body influences and “pass-alongs” that the upper body would sometimes commit to - then try to counter act.
So, watch for you son’s extension, or twist, the upper body too far back, extending the glove arm out to balance himself with a “T” posture and reaching back with his throwing arm, like in the picture below. His youth probably doesn’t have the strength in his mid section (core) to handle what comes next as he attempts to throw/pitch to you.