I had almost the exact same Chiari surgery when I was 12. I had previously been doing fine in school. After my surgery though I was diagnosed with ADHD because I could not concentrate and my grades dropped pretty dramatically. I always was curious if it had occurred because of the surgery, but I don't think we will ever know for sure.
Just wanted to let you know you're not the only one who had experience with it! I hope he is doing well & take care.