autism and gender identity confusion

I have a 5 yr old son who has been diagnosed as high functioning autistic.  He has been receiving a number of therapies and preschool for a number of years.  I am quite happy with his progress in all areas but one.  He is very insistent that he is a girl.  He chooses to play mainly with girls and typical girl toys.  When he does role-playing games, he is always the girl, such as a princess.  He says that he used to be a grown up woman called Joan and that when he grows up this time he will be a woman again.  He gets upset if you refer to him as a 'he', when he is playing the role of a girl.  HE constantly looks for objects to put on his head (like a piece of cloth) to pretend that he has long hair..like a girl.  Is this behavior related to his ASD or is ther the possibility that he also has a gender identity problem?