You'll want to talk with an attorney before deciding whether or not to hire one. This is a fairly minor incident and you may decide the time and expense of fighting it is cost prohibitive. Look at the ticket and see if your son is cited for an infraction or a misdemeanor. If it's an infraction it won't count as a criminal conviction if your son simply pays the fine. If it's a misdemeanor a plea of guilty would result in a criminal conviction and your son would definitely want to avoid that result. Please feel free to call my office for a free consultation.
The police will have some serious trouble enforcing this ticket. Even if the ticket is an infraction, you son should not have to pay for over zealous police officers doing poor police work. Please take this opportunity to punish these police officers for doing sloppy work. Our tax dollars should not be spent on paying police officers to harass law-abiding college students that are not breakign the law. Please call my firm to discuss your options.
I'm not sure how they can prove at trial that it was alcohol in his cup if he dumped it out. If they charged this as a misdemeanor then you definitely need a lawyer. Even if they only charged it as an infraction it might be worthwhile to retain a lawyer. If they are sending him to the downtown San Diego courthouse then it is likely a misdemeanor; if they are sending him to the Courthouse on Clairemont Mesa (Kearny Mesa area) then it is likely an infraction. Many criminal lawyers such as myself give free consultations so it may be worthwhile for your son to contact a criminal attorney to discuss his situation.