You and your son need a lawyer...immediately. You need a civil litigation attorney who has handled litigation on behalf of minors (meaning underage drinkers, not legal minors) in such situations (private party and alcohol served or made available by the homeowners or those over 21).  All of the activity that stemmed from the drinking may or could be linked to the homeowners though I do not think your son can escape at least being partially responsible (i.e., gun was not pointed to his head to drink).  There has been a history of such events occurring in your state and it is considered an illegal act.