Hello, I apologise for the delay in responding.
It is very common for children to see, hear and feel spirits.. their minds are not cluttered like adult minds can sometimes be. I can understand why your son is afraid but it's important for him to understand that this doesn't mean their is a negative presence in your home, only that he has the ability to see... she may have always been there.
Try to show him (while still keeping the "Don't talk to strangers" rule) that what he is seeing does not have to be scary. Maybe find ways to educate him on the subject... I don't know how you yourself feel about the subject but all to many times parents tell children that what they're seeing is wrong or fake... hence many childhood imaginary friends. We do have a "kids club" that may help in educating him further...
http://www.sjgrkidsclub.org/
As for what is happening in your home - I would suggest looking into it further.. by having us or another trusted group come and speak with you... if only to ease your mind... some people see the spirit in their home as a novelty and go on living with this presence but when children are involved and afraid you may need to investigate furhter into the subject so that you can feel comfortable in your home. Most times there is nothing to fear but please keep us posted with how things are and if you need our help we will gladly come and talk with you about how to assist you and your family.