I've had an ADHD diagnosis since I was 7 and I had the same hope that once the brain fog cleared up that I could live without my meds anymore. My celiac symptoms began when I started high school and I'm now a college senior. Unfortunately, I'm going through reevaluation because I'm still struggling in school and inter-personally with ADHD symptoms. That being said, there are a lot of other neurological issues that could be contributing. Our brains use the food we eat as precursors for the neurotransmitters in our brain, so if something is causing issues in that way then a change in diet could help. But that's not guaranteed. There's so much we don't understand about all these processes and every kid is different.
It has really helped me to have an outlet for all this frustration and also to be informed about what is going on and what medication changes are being made, even when I was really young. Medical tests and procedures can be really scary for children especially if they feel like they're not accepted as being who they are even when everyone around them is trying to help. Just from having been on the other end of this situation, I strongly recommend keeping your son in the loop and in the decision making process. It helps treatments work a lot more effectively because they won't be refused as vehemently and lets the child feel like they're also involved and in control of themselves and their lives.