Why was the dad in the trunk in the first place? Did I miss something???

He gathered up some kind of cleaning supplies - I think he was in the trunk cleaning it to remove any evidence. He felt partly responsible for his kid doing something bad, but like an idiot he decided to help them cover it up instead of...I dunno...punishing his murderer or a son. He had no idea they would 1) try to go out that night, 2) instead of looking in the trunk they just slammed it shut and, 3) they pushed the car into a lake.