Yeh, it's likely they'd already been arguing at that point, if that's what happened. Originally, I thought something happened after they arrived at MR's house. Now, I'm wondering if Dylan made it to the house.
If Dylan were already peeved at his dad for not allowing him to go to his friends' house that night (and Dylan wanted to spend the night at his friend's house), I can see the argument continuing in the truck. Maybe Dylan started to text his friend to ask whether his parents could give him (Dylan) a ride from Wal-mart.
Perhaps MR took Dylan's phone then and as they drove along the road toward home, Dylan insists he won't go to dad's; says he'll even walk to his friend's house if his dad won't drive him there. So MR, in anger, obliges and lets Dylan out of the truck with his backpack and maybe some of the food they purchased at Wal-mart. Dylan has left his iPod in the truck, maybe. MR turns off Dylan's cell phone and tosses it into the thickets on the side of the road.
When Dylan doesn't arrive on Monday and dad hasn't heard from him all day, dad begins to worry. He drives to Dylan's friends' homes (apparently doesn't know the phone numbers but knows the addresses) and cannot find Dylan. He doesn't want anyone to know he stranded his son along the road in a remote area at night, so he claims that his son disappeared from the house while he was running errands. He takes the iPod inside the house, and maybe finds some of Dylan's old clothes that he places behind/on the couch to make it appear Dylan had spent the night there.
I can imagine dad waiting until 4:00 to actively look for Dylan in this scenario because he's laden with guilt and holding out hope that he'll hear from Dylan, or Dylan will walk through the door, at any time.
I don't know that I believe this to be the case, but at least it offers some hope that Dylan is still alive.
MOO