http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/53332/2.html This link maybe helpful.
There have been other dicussions here on this. Maybe someone here will give advice on searching.
Here's some ideas.
You can send a DVD of something he is above level in. Does he talk about something in Science? Or makeup great stories?
Can he do some Math verbally?
I printed out AR lists that show above level reading with test scores. This also showed reading volumn.
Chess at an early age can be significant.
I would suggest academic examples more that Legos, Music or Artistic to qualify. But if something seems important to DC/you just include it. Extra is ok.
Has he won any awards? What did he do to earn it.
I think if you explain is talent, you will get more specific to you ideas.
If you have anything from PreSchool or K that was abouve level, I think that is good to show.
Best of Luck!