It's not that uncommon for youngsters to decide things like this. People seem to assume children are total idiots, but really it's far better (to me) that he made the choice himself, rather than his parents pressuring him into it (as OP insinuated falsely). I remember some time back, a boy with quite severe Down's Syndrome won a court battle to go through gender reassignment surgery, despite having the mind of a child.No one can be sure until the time comes.
Although the treatment he is currently receiving means that he can go through puberty at a later date of 15 if he decides to be a boy, so will finish on average by the age of 21 - children can be cruel and it could possibly have a huge effect on his emotional state. I don't see the harm of waiting just a little bit longer - his shoulders are not likely to broaden hugely and his voice will not drop too significantly until he is older and more mature than 11. He isn't even a teenager yet and he is being allowed to make a huge decision and they are carrying it out.
Edit: I believe being in the news will now place more pressure onto the boy to change his gender, in addition to a large amount of people knowing and disapproving of his condition.
Re: bullying, kids will bully other kids based on anything. I got bullied because my siblings are so much older than me. I was bullied because I cried a lot as a child. Hell, I was bullied because I painted my nails black. Kids are assholes, and bullying, sadly, is par for the course of life.