I would do a little bit of waiting and watching.. and if he continues to express that he would like to be a girl, allow him to explore this. It might be normal exploration & confusion of gender, but it could be more than that.

I remember seeing a documentary on this once. It followed three transgendered young boys growing up in three different families. One family completely rejected the idea that their son wanted to be a girl and tried to train it out of him. The second family didn't want their son to become a girl and tried to discourage him but eventually accepted it when he did change genders as an adult. The third family embraced their child's wishes and supported his desire to live as a girl (allowing him to grow his hair long, bought him girls toys, girls clothes, changed his name, etc). Of all three, the first turned out a complete psychological mess who self-harmed and engaged in like behaviours (hard to remember the details), the second turned out ok, with some self esteem issues and a poor (but not terrible) relationship with her parents, and the last one turned out to be a very healthy, happy, well-adjusted adult with a loving relationship with her parents. Just something to keep in mind if he does continue expressing feelings that he would like to be a girl.