It can seem overwhelming teasing out what is precisely going on with kids like your nephew. More info may help us provide more targeted ideas for information and assistance but it seems there are a number of challenges with giftedness, ADHD and a mismatch between academic challenge level and intellect.
You have received excellent advice. There is a lot of overlap between behaviours problems you see in ADHD and gifted children who aren't challenged or who aren't able to perform at their intellectual level due to additional learning difficulties. Feeling frustrated can be at the core for all of those reasons. If your nephew is placed in a learning environment better matched to his learning abilities many behaviours may disappear.
How do figure out what is going on? A comprehensive assessment is really important as you have been advised. In the meantime, observing your nephew in different environments might give some clues. How is he at home? Can he cope with more intellectually challenging material if presented with it there? Does he do any enrichment activities outside of school that hold his interest or make him happy? Is he struggling with any basic skills despite his measured intelligence? It is possible to be dyslexic or have fine motor issues or other challenges that impact on learning whatever his level of giftedness. He is working with an OT. Is it for specific issues?
Well done on seeking out help for your nephew. It can take some time and energy to get to the bottom of everything but you will get there. There are lots of resources. A bit more info may help target them to your particular circumstances.
