Xray have you tried the restrictor? It does help stop the tendency to top the ball through muscle memory. It is just a simple means to an end result at an affordable price. Simply a place to start to try to correct the problem.
If it doesn't do the trick then yes, perhaps professional coaching is the next step, considering a good coach is available to the OP.
Coaching is an answer, but not the only answer to many of bowling physical complex issues that sometimes can be solved through simple means. I know it worked for me and that's why I posted to try to help the OP with a place to start.
Never used it. but it looks like it restricts the wrist as well....
Finding out the source of the problem is key. From the time he puts his fingers into the ball to the time he releases the ball, so many things can cause him to over topping the ball. Or turn the hand early. There are many "hinges" or moving parts in the upper extremity (fingers to shoulder) that can move independently from each other. The body some times will try to compensate when parts of the unit is compromised or restricted. If the hand and wrist are restricted from turning, the elbow and/or shoulder may decide to bump out.
Also, the approach that we take, from 3 step to 7 steps, here the moving body is a moving fulcrum. Many things can go wrong. For example: If a single wrong movement in this fulcrum moves in a certain way (head/upper body moving up and down and up again) or in the wrong time (dipping shoulder or hips during the approach too early), this can disrupt the pivot joint (shoulder) which can cause deviation in the swing thus cause issue with his release.
Then there is the base of the Fulcrum, the lower extremities. If the footwork is off, so will everything else. The strength and the foundation of the bowling approach are the lower extremities. Common cause for swing direction problems is that bowlers may not be crossing over to clear the leg/hip for the arm to swing in a consistent back and forth direction - or in the swing slot.
Finally, the brain. The brain can fool us into thinking we are doing things right. The fact that we think so much already, or have our thoughts focused on one particular part of the game, there are voids in the swing where the brain doesn't tell you, or you don't notice/feel that your hand could be turned to the outside of the ball during the back swing or on the down swing. Maybe here, these devices can help, but does it really help? Finding the source(s) of the problem is much beneficial to the bowler in the long term.
These are just a few things that come to mind. I am no coach, but I know a little bit about the anatomy.
