Your son would be a juvenile under Maryland's law until he turns 18, and this is not the kind of offense for which he could be tried as an adult. The standard protocol in Maryland is that a juvenile would be released after an arrest to a parent or guardian. However, as your son was with family, and was simply handed a ticket, would you seriously have preferred that rather than just issuing a citation and breaking up the beach party that the police physically placed him in custody, took him to the station, booked him formally, stuck him in a cell, called you and then left him there until you could come pick him up? The fact is that with a 17 year old as opposed to, say, a 12 year old, the police have a lot of discretion that they can exercise.
Underage drinking cases are zero tolerance offenses. The days when a bunch of teens on a beach with a couple of six packs was an adolescent rite of passage are over. In today's world it's a strict liability crime, and there's very little wiggle room to fight the statute. At the same time the courts
are fully aware that a criminal
conviction can stunt the long term potential of a young person. The typical disposition for this kind of defense, therefore, is a diversion program in which the juvenile would have to pay some fines, do some community service, take some anti-alcohol classes and serve a brief period of time on probation (generally unsupervised). At the close of the successful completion of the conditions of his probation, the defendant would have all charges dismissed and would not carry the stigma of a criminal record.
All the same, to prevent any unexpected surprises, to make sure that all of your son's rights are protected and to see if the arrest can be challenged or if the case should be tried, you should retain a criminal lawyer for your son.