QuantMech

Both of the courses that gave your son difficulty require applying principles that one knows, in order to solve problems of a type that one has never seen prior to the exams. Some high schools provide a much better background in work of this type than others.

What I would recommend is that your son should obtain the high-school physics book written by the Physical Sciences Study Committee, a group of faculty headed by Jerrold Zacharias of MIT (quite a long time ago, but there have been multiple revised editions, and you can purchase it from Amazon, or possibly obtain it from a library near you). If he works through the entire book this summer, about a chapter per week, he will have an excellent understanding of how to convert basic principles into equations for particular situations. The PSSC Physics book includes many problems that do not have corresponding worked examples--that is one of its principal virtues, in preparation for university physics. Also, your son should find someone who actually understands physics to help him over any rough spots.

It's perfectly possible to bounce back, but it may require taking a quite different approach than your son has been accustomed to take, with his high-school courses.

Also, working on the physics will have excellent carry-over for calculus, because it helps to develop the same thinking style--even though PSSC physics is not calculus-based.

One other thought: Was the physics course electricity and magnetism? If so, it might require multi-variable calculus to really understand; then the advice above is still valid, but your son should seek advice about how to time the re-takes (if there's any option).