I guess it depends on what you think is normal - I believe most boys and girls go through a stage of playing at being the opposite sex and this can last over a period of years, but the majority grow up to be heterosexual.
Sometimes when this behaviour continues especially after puberty then the individual does go on to be either homosexual or have gender identity issues. But this would be more rare. Having said that, this behaviour can be noticed by other children. My son came home from school when he was about 5 and told me that a boy in his class "really" liked playing with the girls and their dolls - he said that most of the boys did like playing with girls but that the way that this boy played was strange. He still gets on fine with this boy (they are now 8) but he still says that he is a bit strange.
I have found one of my sons trying on my shoes and the other messing about with my underwear......but that's a different story!
Personally, I would leave him in peace, but if it persists or starts to effect him otherwise (e.g.getting bullied) then you may want to get some advice. At the end of the day his happiness and the acceptance of him for who he is are probably the most important things. But the chances are that it is just a phase.
Do you have any other kids?