He did the crime (actually several), he must do the time.
If he wants to play big boy games then he must accept big boy penalties. Fuck your PC "Oh but he's a kid with his whole life ahead of him!" bullshit, he's chosen his path, let him reap the consequences.
That's just a straw man argument. The actual problem with treating him as an adult is that that is contrary to fact. He is not an adult.
In the state of Georgia a fifteen year-old cannot vote; he cannot purchase liquor; cannot obtain a driver's license, cannot hold a full-time job. The rules we have for minors assume they're incapable of making adult choices. It's logically inconsistent to believe minors are not competent to make responsible decisions, but then claim we should treat them as if they can decide responsibly because they've failed to do so. When have you ever used reasoning like that for anything else? I had a housemate once who decided to become her own herbalist. She went to the herb store and bought a lot of herbal shit and promptly made herself sick. By your logic I should go to her for medical treatment because (a) I previously had reason to believe she was not competent to practice medicine and (b) her subsequent actions proved my suspicions correct.
You don't need some namby-pamby PC mumbo jumbo to know that most teenagers have a penchant for doing spectacularly stupid things, but that *most* of them grow out it. That's common sense, and the law should take that into account. And science actually backs up common sense here. Most people's brains go through a development spurt in their "executive functions" (acting according to long term plans, inhibiting impulsive actions, directing attention) when they're around fifteen. That means there's roughly a 50/50 chance someone under sixteen is neurologically incapable of not acting like a jackass.
So both science and common sense tell us that treating children as if they were adults is irrational and serves no useful purpose. That doesn't mean you do nothing when kids commit crimes. That's a false dichotomy. It means you do something different.