My adult son's got into a fight at home.No one saw who swung first because no one else was in the room but the two of them.

This is simply a classic controversy between people calling 911 and seeking police intervention on family members and then later, when head clear, want to drop charges, only to find out that they dont want the results. You need to be VERY careful in this situation as this sounds more like you are trying to keep your one son from having to deal with criminal charges - and that may backfire badly as there is no question that a criminal batter occured, law enforcement showed up, and in theory BOTH sons could have been charged and may well be if this goes sideways. There is also a potential that you could be charged with obstruction or filing a false report by trying to retract what was claimed by law enforcement, who candidly have zero motive to have lied or exaggerated on the issue as they simply ould have charged BOTH with domestic violence/battery. To be clear - to law enforcement efforts to retact criminal charges after a domesitc event is so common that most locales make well known that if the victim sabotages the prosecution, THEY will catch charges, so people stop calling 911 and using law enforcement as DR. Phil.