I've read several posts that strongly encourage that if you have a NROTC scholarship and school as plan B for USNA, that you notify both the school and NROTC that they are back up and that they are OK with that arrangement. Almost all NROTC units and colleges will work with the candidate and have no problems, but some issues have come up. That is why you want to inform everyone so they are all on the same page and agreement.
If DS doesn't show up for NSI, then he will definitely loose the scholarship, but it also is not fair to take a spot at NSI when he has no plans to go to NROTC. He should contact the unit so the date may be pushed back till after I-Day or held, then he has his insurance policy, but doesn't have to go to NSI or take a unused spot.