If the goal is to decrease family conflict and/or change the identified patient's behavior, I'm not sure that trying to use socratic dialog to help him to see and acknowledge the perspectives of others is a very promising approach. Developmentally, I wouldn't expect a 12-year-old to be very good at grasping the perspectives of others and taking them into account. If he is immature and has ADHD, the idea that he would change his behavior because he can acknowledge the perspectives of others seems even more unlikely.

You haven't said anything about how the parents have been dealing with this situation thus far and whether they have been able to deal with his behavior effectively. I assume that they haven't been able to deal with his behavior effectively since they probably wouldn't consult you if they could.

How about starting with a more behavioral approach (such as Barkley's approach to dealing with behavior problems/conduct disorder/oppositional defiant disorder). Suppose the parents could do an effective job of reinforcing appropriate behavior, setting clear limits, and enforcing limits effectively. In addition to improving the identified patient's behavior, it would reduce the level of conflict within the family, it would increase positive interactions, and it would reduce negative interactions. It would also provide a great opportunity to work on communication, on anticipating the consequences of actions, negotiation, problem-solving, etc. One part of working on communication, negotiation, problem-solving, etc. could be working to see and acknowledge each other's perspectives.