A micro phallus? I know "it's not my business" but I read that there are things that can help. A series of testosterone shots can cause more growth during puberty. He is 7 now and is not shy about walking around naked after his shower so I saw it. I don't think he even has enough there to hold on to when he pees!
Edit: Seriously people. Get your minds out of the gutter. He has an honest medical issue.
Another edit. It can indicate other issues. One of which could be he has female genitalia too. How hard might it be for him if he is supposed to be a she?
Another edit. I really not impressed with those of you that read the first sentence or 2 and make a post. Short attention spans?