Try taking the throwing arm out of the equation and work on the release. Put the elbow of the throwing arm at a 90 degree angle out in front of his body with the elbow facing the target. Put the below of the throwing arm resting on his glove hand.nwhich reaches across his body Practice that 90 degree range of motion with the release point, making sure he keeps his hand behind the bal instead of allowing the ball to roll out the side of his hand.
It’s like a flicking motion but allows the hand and wrist to get the feel of the proper release of the ball. I bet the above comments are correct about his grip, but I also bet he is rolling his wrist at some point and the ball is sort of rolling out of his hand and he isn’t feeling it come off his finger tips.