I remember when I needed glasses when I was a kid. My mother picked out the frames and I hated wearing them. It wasn't until I needed a new pair that she allowed me to pick out a new frame. I took a friend with me and we picked out a pair together, so I felt that they were stylish and I had an outside opinion besides my mother. Your son may be teased, he just doesn't want to tell you or maybe he just doesn't like the frames. Kids at that age are trying to assert their independence and don't like to talk to their parents about anything, even if the parents can help. Luckily, my eyes corrected themselves, and I don't need to wear glasses now.
By chance, have you tried maybe a bit of psychology. Possibly telling him that if his eyes don't improve by wearing his glasses that he won't be able to get his license at 16 because he needs to pass the eye exam.
Also, could he wear contacts? It's an option he might be open to.