Is he trying to be a Navy nurse? That's a different type of OCS at Newport, RI, where they direct commission doctors, nurses, lawyers and other staff corps.
https://www.navy.com/dam/Navy/Navy-IMG/Downloads/pdf/healthcare/nurse-brochure.pdf
http://www.ocs.navy.mil/programs.html
The needs of the Navy will govern selection.
If he doesn't want to be a nurse (staff corps) but a line officer (warfare), then it is indeed OCS, also at Newport, RI. OCS is very competitive and depends, as always, on the needs of the Navy. Consider it as the adjustment intake for whatever new officer endstrength it needs, to add into USNA, NROTC, enlisted commissioning program numbers to hit that FY's target range. The Navy can "shop" through OCS applicants to find just who it needs to meet various goals.
Your DS' BSN will have the attractive STEM element. He will also have some knowledge of the Navy's mission from exposure to his sister, which I hope he mentions as a source of inspiration for service. If trying to go Navy nurse, his actual med-surg work experience will be good.
It's a more compressed application - transcripts, DODMERB stuff, personal application. Am not current about letters of recommendation or interviews. No nominations, of course.
There is no way of predicting who or how many will be successful. I am an OCS grad myself and count myself very fortunate to have been chosen. USNA and NROTC (gradually) opened up when I was halfway through college, and the nice staffer at my MOC's office told me when I called about USNA that "you want to marry them boys at the Academy, not be one, and Rep X isn't inclined to nominate the fair sex." But that's a sea story for another thread...
Best of luck! Let us know.
