Son Charged With MIP For Being Near Open Containers

My son is 19 and attends a local college in SC. Last weekend, while exploring with 2 other under 21 friends, my son and his friends got a MIP ticket for being at a party that had open containers. none of the boys were drinking or did not have any alcoholic beverage in their posession when the undercover cop flashed his badge and started writing tickets. My son confessed to the officer that he and his friends had had nothing to drink and challenged the officer to give them a breathalizer test and the officer refused. According to the law as I interpret it the accused person under 21 must buy or knowingly posses to be in violation...the cop said they were in violation because for the open containers didn't add up to the amount of 21 years olds in the room at the time.
Did the officer violate my sons rights by refusing to allow him to blow? Can they enforce this base on officer refusal?

Your son can request a breathalizer test, but the officer doesn't have to give him one.

In some jurisdictions in South Carolina, minors don't necessarily need to be holding the beverage. This is called "constructive possession", and they can be charged with the same as if they were possessing it.