Yes, I think you are over-thinking the importance of the nominator. I think it's a good idea for it to be someone who works with kids, and who "gets" your son. Family friends should be fine, or a teacher, or a librarian, or almost anyone who's not a relative. If you have teacher friends who are able to say things like, "When he was five, I observed him reading The Lion, the Witch, and the Wardrobe to himself and he was able to discuss it with me," then use them. (That's a made-up example to give you an idea of what observation of out-of-age behavior might look like.) But if you have test scores, I really don't think the nominator piece matters much.
