Aside from potential credit/placement? None. But depending upon the exam, this could be a big benefit, and given that a HS sophomore has no idea where he’ll be attending college, is not one that should be quickly discounted.
Which brings up the question: why would you advise him not to take the exam?
Many BS do not offer APs, but offer courses that contain the same content without the label, and many students still choose to take the AP exam.