What year did he take the Math 3 course? Was it last summer 2021?
Here is more information regarding the UC’s and Pass/Fail for A-G courses:
Courses in which a Pass or Credit grade has been earned cannot later be repeated for a letter grade.
Pass or Credit grades are assumed to represent achievement at the C- or higher level, regardless of what those grades mean at individual high schools.
UC-approved-honors,designated AP and IB, and UC-transferable college courses in which a Pass or Credit grade is earned will not receive an extra point in the GPA calculation.
A B grade in a 6 week summer Math course is very acceptable. I would highly recommend taking the letter grade especially if the class was taken this summer 2022?