Light airplanes are extremely easy to steal. There is no key to put in the ignition--once you get inside (and let's be honest, the door locks suck and are easily defeated) you can start any of them up if you know how. Some do have aftermarket ignition locks but those aren't very common.
I don't really buy the mothers claim that he had been given permission to fly the plane before. If he was a lesson or check ride away from his PPL then he was most likely endorsed by his instructor to fly solo but in a single engine aircraft make and model specific. I don't know that I've ever heard of someone getting their initial private in a multi. Second even if he did have the logbook endorsement from his instructor he's still limited to VFR (visual) conditions so the bad visibility and low clouds cancels that part out too. I'd hope an instructor didn't give this kid a night flying endorsement because that's just a death wish in and of itself.