Shooting Accuracy: The SMALL Stuff is VERY Important
The longer the shot, the more attention is needed for proper mechanics. Here's a good example of how a small mistake turns into a large mistake.
Roll a basketball along a line on the gym floor. If it is off line just a little, the farther it gets from the starting point, the farther it deviates from the line.
It's the same with shooting. If the ball is released just slightly off line with the center of the basket, the longer it is in the air, the more it will be off line. When you consider that the margin of error left and right is only 4.5 inches, it is easy to understand the importance of a perfect release.
With that in mind, practice often by focusing on the index finger while modeling the shot - while watching TV, in the lunch line, anywhere - and while shooting air shots. When focusing on the index finger, THINK about moving both the index and middle fingers together. Why not focus on both the index and the middle fingers? It's impossible. THINKING will make both work as one.

Comments
I have him wear the strap, I have checked to make sure that the ball is not resting on the heel of the hand. I have also had him focus on releasing the ball off of the V during air shot segment. Could the ball be dropping back down on the heel of his hand at release? Any suggestions?
It's a timing problem. The ball has left the hand before the fingers snap down. How to fix it?
1. Balance the ball in the shooting hand. The palm should be facing up. Slightly move the BASE of the Thumb and 5th finger toward each other. This creates a little more pressure on the ball from the tips of the tumb and little finger. This SLIGHT pinching action raises the ball off the palm.
2. With the non-shooting hand hanging along side the body - extend the shooting hand high so the shooting arm is completely extended. The shooting hand WRIST will be flexed completely back so there are WRINKLES in the skin.
3. Flex the knees SLIGHTLY and explode upward while at the same time fully extending the wrist. The power the makes the ball move upward into the air should come mostly from the hand and very little from the legs. The ball should be spin backward. Do this 5 times.
4. Repeat the same procedure but this time bend the shooting arm at the elbow SLIGHTLY.
5. Repeat the same procedure but this time the elbow is dropped to the level of the shoulder. Repeat 5 times.
6. Once a backspin is consistent, move to the basket. While in the same position as #5, shoot 5 shots ONE step out from the center of the basket. Then two steps.
IMPORTANT: Anytime the ball fails to spin backward, perform 5 perfect airshots with backspin in a ROW from the #5 position. Then go back to shooting at the basket from one step away, then two steps away. Making the shot is not important. Shooting with backspin is the objective.