@SCubb First - congrats to your DS and family!
My DS was in a similar albeit a little bit different of a boat last year - he was one of those rare ducks who wanted ROTC, didn't apply to SAs, wasn't interested when Annapolis saw his NROTC application and recruited him), so in my DS's case it was an easy YES to call the top school with scholarship in hand, express interest, and continue to build a relationship. And yes he went on-site to meet, shadow a midshipman, etc.
Can it help with admission - the real skinny? The official line is that the applications are separate/ distinct. But they don't have to be, and in some cases are not. Short answer is YES they ROO/ PMS
can go to bat and advocate with the admission powers that be and
YES it can absolutely help. There is no guarantee but last year for my DS they did just this at one of his top choice schools and we had a change from wait-list to accepted 5 days after a letter was hand delivered to the University president and director of admissions from the PMS. Just be aware that unfortunately, some of these leaders are a little bit leary/ some have been burned by kids who really want a SA but decided to pretend to be all-in interested in the school- if they get even a whiff of that, they won't lift a finger to help. So just be honest as I'm sure you already are, and yes I would reach out to build a relationship.
Ultimately *but not yet* I recommend an on-site shadowing at the top 2 or 3 choices - DS and I learned an awful lot through these. really get to know from the midshipman perspective, stay in a dorm, eat at the dining hall, etc. So maybe that would be when you pay for the planeride- in the spring. It solidified his decision - was super-informative. I think they do this at the SAs sometimes too - not sure.
Getting on a plane? IDK. in your case, I wouldn't yet. I would certainly call, and point out that you had boots on ground and met with their ROTC unit in an orientation meeting or whatever. I'd have your son make the calls /emails and build a relationship (read: not you, him). I might hold off on that planeride until you hear from the SA and rule it out, or if and or until your DS decides he wants the school as his path. Hopefully he'll get into the SA - I don't know him, so don't take the comment as a knock. In your DSs case, IMO it's a little tougher to make that investment/ ask for influence on admissions as you're interested, but not committed. It's like a the old pig and chicken who open a breakfast joint together. The pig is committed (Bacon), and the chicken has a lower level of all-in by laying the eggs but walking away. Right now for this school you're a "chicken" not a pig. but I would just recommend being honest and expressing that your plan A is the SA, but that if he ends up going the non-SA route, he's coming if they'll have him.
IDK if that advice will connect with you but I hope it does. I suspect others may offer that the PMS/ ROOs cannot influence admissions. To which I say, simply - that's nonsense- for a fact, they can, they may, and if they chose to they can influence greatly. Good luck.