When I was considering applying by portfolio, I came to the conclusion that video/audiotape were THE ways to go! If you see/hear an HG+ kid in action, it's pretty clear that's what you're seeing. And since even the most PG kids aren't usually writing novels in 1st grade, I don't know how else you can show what they can do.
Lists of above-grade-level books read and in what time span they were read is a good idea. (Reading one Harry Potter book over the course of a year is less impressive than reading a Harry Potter book in a week, for example.) Have him dictate a story to you or talk about something he loves and film it or tape record it. I'd recommend providing a transcript of anything you tape as well, just to make things easier on the DYS evaluators.
Good luck!
