Depending on the age of your son, Idlewild park here in southwest PA does a very nice Halloween event on weekends in October. It's really tailored for the younger kids (I'd say 8 and under) but is fun and well done. The park site is
www.idlewild.com
There are dozens of very nice campgrounds in the area and many local attractions other than the park itself. I understand that you want to stay toward the northern part of the state, but this one is worth the drive if you have the time.
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