Going to The Citadel is a huge financial commitment... and I understand your trepidation. I'll try and break this down into a few different sections.
Transfer scholarship to The Citadel
If this is something that you seriously want to do then you should be contacting The Citadel's Admissions and AFROTC departments right now. If I remember rightly from my days as a cadet there is a fair amount of competition for good/eligible scholarship recipients. You should get on the horn as soon as possible.
Going in as a freshman and earning a scholarship
This is a little bit harder to estimate. He would need to have good grades. Good is a relative term because the military lifestyle hits the GPA by 0.25 to 0.5 points at a minimum. Secondly his major will impact his chances as well. A EE, CE, or CSCI has a much better chance than your run of the mill PoliSci or BADM major. I know this because my own AFROTC scholarship (4 year, Type 2) offer was made mostly because I was in the CSCI program. (My DODMERB found me medically ineligible though.)
I'd also like to note that all of my friends who wanted scholarship received 2 or 3 year contracts from the service of their choice.
The Citadel graduates something near commissioned officers a year. This equates to ~40% of the graduating class. Not everyone, however, has a scholarship. (And not everyone competes for one: some people decide to commission a week before graduating.)
Either way I'd be a bit of a helicopter parent on matters of finance because, as everyone knows, there are few other aspects of cadet life you'll be welcome to do that with.