This is a good article that touches on the subject:
http://talentdevelop.com/articles/sexhighlygftd.html
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Setting appropriate limits for any teenager is difficult enough, but what rules and guidelines should parents set when a child is chronologically 16 years old, and has the cognitive reasoning of a 25 year-old adult?

I would explain to your son that it would be a mature and caring act for him to look at backing off from this relationship in order to protect her from some potentially devastating consequences.