You have a gifted family. He is gifted. Highly. Probably profoundly.
He might not want to do the test because someone has to convince him why he should and why it matters. Some gifted kids feel offended that they've been asked such easy questions and refuse to do the test.
He will do better finding someone older than himself, as similar interests would help.
Mentors, professionals, in subjects that he is interested in.
I'm not much help... but do get him tested with someone who is experienced working with gifted children. Typically they may be able to convince him to answer some of the easier ones, and that it will get harder, etc etc.
You're not too late. Keep at it.
Take care.
