Your son needs to report this to his NROTC class advisor immediately. Failure to do so is choosing to hide this event from his chain of command, which will be viewed in an incredibly negative light; any interactions with the police or the courts need to be disclosed to the chain of command, period. Units have a way of finding out about events like this. However, do not waste time or money on an attorney. Your son is not the first midshipman to receive a MIP summons. Community service or whatever else the court requires to get this expunged is commonplace. He will also face consequences from NROTC, most likely starting with a Performance Review Board. As a result of the PRB, he could face probation, leave of absence, or disenrollment; it has implications for the scholarship. However, assuming this is a first offense and there isn't more to this story than you're offering, him coming forward and admitting his actions will most likelay translate to probation(no impact on benefits). From that point on, he'd absolutely have to keep his nose clean and work hard in the program.
If he doesn't disclose it to the unit, maybe they don't find out. But if they do, the consequences will be significantly more serious than if he admits his actions. Integrity is one of the cornerstones of this program. Intentionally hiding this from his chain of command is the opposite of integrity.
As mentioned above, students go through security clearance processes during and after NROTC. Tickets, summons, arrests, etc. are thoroughly investigated. Lying now, and having it discovered later, can sink a student/officer; I've seen this before.
Honesty is always the best policy. Taking responsibility for ones actions are part of NROTC, and also part of growing up. If you help your son hide this event, what kind of lessons are you teaching him? If your son doesn't do what's right, what actions will he take in the future when more is on the line?
He needs to talk to his class advisor, bottom line. If he were my midshipman, I'd appreciate his honesty, and I'd keep it in mind as we took further action.