2 separate issues:
1) Life/DUI: a conviction here is devastating to employment prospects within any conservative business, any government job, etc. Just about the only jobs that don't care about DUI are jobs that either don't pay very well, or are physical and dangerous in nature. Having this DUI charge reduced or dismissed is critical! Even insurance companies won't allow a company driver to be on a policy with a DUI within the past seven years, so no driving jobs either. Having said that, drinking and then driving doesn't mean DUI... it is possible your son was under the legal limit in the State where he was arrested. Don't convict your son in your own mind until the facts are straightened out.
2) Office Accession: the *only* ray of hope is that your MSIV is of legal drinking age. Everything else points to immediate (within the next three weeks) separation from the ROTC program. If I were your son, I would tell the PMS exactly what happened without drawing any conclusions about DUI or not DUI. Don't allow your son to convict himself any more than you should convict him before all the facts are clear. i.e. "Sir, I had four beers over 2.5 hours, and I am certain I was not legally drunk. I wouldn't have gotten behind the wheel if I had felt I were legally drunk. I realize this was bad judgment, because even though I don't think I was legally drunk, I shouldn't have gotten behind the wheel after drinking four beers."