I would just force contact and drop into the Unit. They will talk to you. Most Units require a separate College Program application that is due by June or July at the latest. They have to run the apps by Pensacola so they can screen them. Not an extensive app but yet another app. So you would definitely not want to just sign up for the classes. Every Unit handles the class registration deal differently for cross-townies. You'll need to check with the Host Unit.
The Navy has published numbers for sideload scholarships for your son's class (Year Group 2017). Nationwide, they will offer 93 3-Year Navy scholarships (offered during Freshman year) and 119 2-Year scholarships (offered during Sophomore year). From NSTC Note 1533. Not sure about the Marines. Those numbers will change based on whether the expected wash-out rate materializes. Those are actually pretty high numbers. This year's graduating class was offered zero sideload scholarships because they offered too many 4-Year scholarships. The class of 2013 was offered 2254 4-Year scholarships but this year's incoming Freshman class will be offered just 1207. They are doing that so they can essentially reserve more scholarships for College Programmers that deserve them. Makes sense. They can get the kids on campus and make personal evaluations instead of choosing from data presented to boards. So if DS does well, ie. everything they tell him to do, he has an excellent shot of getting a scholarship. To be competitive, he will next to take the same Calculus and Physics courses the Scholarship kids do. The Unit can tell which courses those are. He could take Calc, but take the wrong Calc, so ask. By and large, the Mids themselves don't care if you're College Program or Scholarship. In the fleet, it never comes up. You're either Academy, NROTC, or OCS.