It is possible that your child is transsexual or transgender. No doctor would change him into a girl at this age. Help him to find interests that occupies his mind, so he can leave this complicated issue until he gets older.
Note that wanting to try out a reverse gender role and wanting to actually change gender is different, there are people who want to try out a reverse gender role but who normally keep to their original gender. Note also that wanting to try out a reverse gender role need not mean a wish to do any sexual acts under this reverse role.