If he slipped and fell in and had his phone with him he would have no time to call anyone. I just think he thought he would be back in plenty of time before his dad got back from running errands.
IMO
This is one thing I've thought ... packs up stuff to go to lake, maybe puts his phone in his pocket (possibly not realizing it isn't on, if Dad had turned it off night before), walks to lake, pulls phone out of pocket, drops it in water, tries to find it ... with backpack still on. Backpacks will change your balance point, obviously, so it isn't far-fetched to think that he loses his balance and falls while bending down (I've done it with a pack on).
Also, I'm sure the lake is low, as are all lakes around here. I have never been there, so I don't know how it would go, but assuming the lake is much lower than it used to be, it might drop off to the deep part quite soon after the shore. (I don't know what that is called, when the shore is gradual and then it sometimes just falls off.) He was familiar with the lake, but possibly not in this condition, and maybe was surprised by how quickly it got deep, and got in literally over his head before he realized it. Yes, it's been warm here, but the water is very cold and it wouldn't be long before he was incapacitated.
Again ... I am just speculating. It is unusual that his phone would be off, but I can think of a lot of reasons that it would be off that don't include some of the more nefarious scenarios.