For the NROTC scholarship, there will be a separate report date for the New Student Indoctrination (NSI) training for new scholarship recipients that differs from the Fall semester start of school. NSI is located in Great Lakes, IL and can be thought of as Plebe summer for NROTC mids.
Each unit may also hold a short NSO (New Student Orientation) that is different from NSI. These are usually a few days right before classes start, and will just be the new 4/C mids coming into that specific program with the school.
As for whether to hold on until I-Day to let go of the scholarship? That is a bit of a pickle. If your son is concerned that he might get injured and be unable to report to the USNA, remember that it is the same DoDMERB group that regulates the medical clearance for both. If there is something that happens to cause him to be unable to report for Plebe summer at the Academy, it might also be something that will not allow him to report to NSI and keep the scholarship.
I believe there are some threads on the forum about what to do when a candidate has multiple options. Try the search function and see what comes up??