If he expected to be back before his dad arrived, then why take all of his belongings with him?
And he couldn't have gone to the lake to fish then, because that is 4 miles away.
So I can't figure out where he would go, while taking all of his stuff, and planning to be back by 11.
The creek isn't that far away, and nobody said he'd have to go to the lake. He left some clothes at the house, maybe he brought the backpack along because of something else he wanted with him.