I have a similar situation. My oldest was stereotypical, early developing HG+ kid. Her younger brother talked and walked much later, didn't read while he was still in diapers and generally seemed like a more 'normal' developing child. Definitely bright, but nothing too far out of the norm (which was fine with me).
As he got older, I noticed that he seemed to grasp really abstract concepts and that he was fiercely curious about the world around him, but in a hands-on kind of way, where as his older sis was more passive and bookish. We had him tested as a 4 year old to see if he would qualify for the gifted program his sis attends and got fairly average scores. I had him tested this past spring by a 2e specialist and got much, much higher scores, quite a few of them at the ceiling of the test. One of his processing subtest scores was in the 5th percentile, so there was quite a scatter.
He has the same tracking insufficiency problem that your son was diagnosed with. The tester also suspects ADHD and dyslexia. He is definitely gifted.
My recommendation, find a psych that is familiar with gifted and 2e kids. It is tricky to test kids who are only HG. When you add LDs into the mix, it can be really difficult to get an accurate score.
