Keep in mind that by the time he started the second half of the class in January of his senior year, he wouldn't have been thinking about physics since the previous summer when he wrapped up the first semester summer course. If he didn't retain much, the second semester could be really difficult, and even as a second semester senior, you don't want to run the risk of a C/D/F in a class that you'll still have to report to the colleges that have accepted you and you're planning to attend.