First, your DS should get an opinion from his RC. Not all QNS are the same. You DS could have barely qualified or was a high qualifier. Assume you DS gets good grades and improve his SAT/ACT at a prep school or college, that should get him in next year.
Why do you say if he accepts ROTC scholarship, he can't reapply to West Point?
Personally, I prefer a regular college with ROTC over a self prep option. I, not to offend anyone, don't see any proof that a prep school increases someone's chance of getting into West Point over a regular college with ROTC. I worked with a kid that was either QNS or didn't complete the application several years ago. I was surprised he didn't get in. He figured out he needed to focus and needed the discipline, so went to Valley Forge and got into West Point next year. Didn't even have to retake the SAT as his scores were off the chart. I believe he would have still gotten into West Point even if he went to a regular college, because he knew what he had to do after his rejection.