You didn't say what he'd be using it to hunt. Assuming it's Elk or other large big game, if he shows he can handle it, more power to him. I'm not sure if most 15 year olds would be ready for that powerful a rifle. Not saying some can't. I imagine he'd get to know how to handle it after the initial sight in period, which is probably the most challenging of any heavy caliber rifle.
Otherwise if he's primarily going to be using for game that doesn't need a magnum caliber, your experience and guidance would be good to help steer him toward a caliber that is more in line with the game animals he's hunting.