Unfortunately, your son needs to bite the bullet and contact the university directly and explain his situation, just as you have explained it to me. He must ask if his acceptance will be rescinded if he drops the course. In cases like these, it's actually more common for an acceptance to be revoked if the student drops a challenging class like AP Physics without telling admission folks than if the student continues with the class and gets a D in it, as long as the student has clearly made an effort to do better.
Thus I suggest that he does this:
1. Calls the university and asks for the name and email address of the admission staff person who oversees applicants from his high school (if he doesn't already have this information).
2. Writes to that official and explains his situation
3. Follows the advice he is given re continuing or dropping the class or moving to regular physics
If he doesn't get a response to the email within several days, he should follow up with a phone call. I advised that he should begin with an email so he will have the response in writing. But, ultimately, he may find that a phone call is the most efficient way to reach his admission rep, especially at a big college.
You do have to steel yourself for the possibility that this single low grade could have an impact on the acceptance, merit grant, or honors program invitation, but I still feel that your son is better off explaining the situation now and seeking advice from the university. As I pointed out above, if he tries to fly under the radar by dropping the class without reporting the change, it's likely to hurt him in the summer when the college gets his final transcript from his high school counselor. So it's better to deal with that possibility promptly, while there's still time to make other choices if necessary … although hopefully it WON'T be.