Well I think the only proof you need is the fact that everyone who knows your son knows full well that he is never without his cornrows, rather than looking for technical proof in the photo itself.
Having said that, I'm afraid I just don't see the big hoo hah, I mean I always wear my earrings but I wouldn't be upset about a photo being taken without them or them being edited out if the photographer felt they were distracting
OK you may feel it doesn't show your son as he normally looks, but hairstyles are cosmetic, + photographers routinely make cosmetic changes to photo's. It's a sports photo, let your son enjoy his fame, with or without his cornrows, without turning the photo thing into a drama, is what I say
Best recourse, if you feel so strongly about it, is to request the photographer to put them back
If he/she refuses, you could always revoke your parental permission for it to be displayed on the internet + simply ask that the picture be removed from the website.