This is a good article that touches on the subject:
http://talentdevelop.com/articles/sexhighlygftd.htmlSetting 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.