I have worked with teens for 30 years now; and you can never be effective with parents in the room like that. I would suggest that you state that it is important for you to see him alone for most of all of the visit. You can get input from the parents at the beginning; and give them some feedback at the end. But if that’s not possible, I would ask them to find a different doctor.
Not sure if there is any other way, really. If you don’t do that, you will just be trying to dance around the issues and not really addressing the causes and not really connecting to him.
Sorry, Howard