Thank you for your replies.
SonofShadows - I think your dealing with some entity (human or otherwise, I'm not sure) that is capable of hiding in your blind spot. This basically means that whenever you look at it, it appears to vanish. It is still there, you just don't notice it. I suggest looking into this. However, it may just be your guardian daemon/angel/spirit.
Rawrx2 - I see no reason why that person wasn't an angel. I've seen them before and it makes sense that they'd be at 9/11. Interesting fact, on 9/11/01, all the random number generators in the world (used for probability experiments) stopped being random. I don't know the details, just that the numbers generated stopped being random.
hippiewitch - A lot of children's imaginary friends are spirit guides, we just stop noticing them when we "don't need to anymore" or our parents just tell us they aren't real. That kid you mentioned probably is his spirit guide. I find it slightly unusual that other people can see him too, but that is perfectly normal. If you son won't talk to you about his guide, you can contact him yourself (don't ask me how).