L.L.
Hi, J.!
I don't know what will help your son break his habit. I do know that a thumb guard didn't stop me. I was dealing with childhood trauma so it was more than simply a bad habit. I depended on it for soothing. My dad tried everything to get me to stop. So I learned how to hide it and only did it when I was by myself. My teeth survived and I did not end up needing correction. My heart goes out to you and your son. I hope you find a solution. Best wishes:)