Did they pull off the road? No one drove by to see this grown man beating his 5'0 son to death? If MR was in a crazed fit of rage, do you think a kid would willingly get out of the vehicle? Did MR drag him out of the vehicle?
I have thought they got out of the truck too. Actually, what has occurred to me is that dad got mad at Dylan for not wanting to be with him, told him to get out, threw his backpack out with him, told him to find his own way to his friends, drank some in his truck, got out himself, chased Dylan, and Dylan lost the battle. No evidence in the truck. Then MR spent all night and most of the next day figuring out his story, which included a missing fishing pole.
But of course we don't know for sure, but this scenario would fit IMO.