It is perfectly normal for your son to experiment with female attire at his age. He may even want a play kitchen set or even see what it is like to hold a doll and play with it. However, I would not keep these items usually reserved for girls around him for prolonged periods of time. Keep the usual boy related toys around for him to experiment with and have fun playing with. When he does dress up as a princess, which I hope that you are curtailing, just tell him that it is alright to play dress up but that he is not a girl. When my sons were younger, they loved to wear my high heel shoes at home until about the age of five. My husband was mortified but I knew that it would not do any harm.
Your son is only experimenting and I would not be concerned for his age. However, if more female affinities are developing and he is around the age of 5 or 6 when he starts elementary school, then you may want to discuss your observations with your son's pediatrician.
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