Hello Granny,
This is not typical behavior, though I am not sure that there is great cause for worry at this point. Freaking out (if you can avoid it) is understandable, but probably not the best response. However, making sure he understands personal boundaries is important. Its fine to be curious about things, but he needs to know that it is never acceptable to take other people's personal items. Your daughter may wish to consider locking her room for a time.
It sounds like your grandson has some issues to sort out about how bodies work, sexuality, and differences between males and females. I recommend finding a psychologist for him to see (in this case a doctoral level professional). I would recommend a male therapist who is very gentle and non-judgmental. Best bet for your grandson's well being is to avoid working with anyone who will shame him. You want him to be able to work out his issues/curiosity, and if he is driven to shame he will just hide these feelings. Hiding such feelings and questions will not make them go away, but could lead to greater problems in the long term.
Best wishes
Rebecca Resnik
Thank-you so much for your help.I will pass this along to my daughter.